The majority of the humor in The Importance of Being Earnest stems from dramatic irony: the audience is always aware that Ernest does not exist and that Jack and Algernon are both pretending to be him, but the other characters are not. This dramatic irony is at its most palpable during the two parallel proposal scenes.
In Act 1, Part 2, after Jack has proposed to Gwendolen, she reveals that she has always wanted to marry a man named Ernest:
Jack: But you don't really mean to say that you couldn't love me if my name wasn't Ernest?
Gwendolen: But your name is Ernest.
Jack: Yes, I know it is. But supposing it was something else? Do you mean to say you couldn't love me then?
Gwendolen: [Glibly] Ah! that is clearly a metaphysical speculation, and like most metaphysical speculations has very little reference at all to the actual facts of real life, as we know them.
The audience, who can easily perceive Jack's agitation, knows that his question is not a metaphysical speculation but a genuine question. Jack's slip of the tongue later in the conversation is also insignificant to Gwendolen but quite meaningful to the audience:
Jack: Gwendolen, I must get christened at once—I mean we must get married at once.
In Act 2, Part 2, Algernon and Cecily have a very similar exchange:
Algernon: But, my dear child, do you mean to say you could not love me if I had some other name?
Cecily: But what name?
Algernon: Oh, any name you like—Algernon—for instance...
Cecily: But I don't like the name of Algernon.
As in the first proposal scene, the audience is aware that Algernon's name is not really Ernest and notices when he later misspeaks:
Algernon: I must see him at once on a most important christening—I mean on most important business.
This line is also ironic because the audience has just watched Jack make an appointment with Dr. Chasuble to be christened and knows that both men are intending to change their name to Ernest.
The scene in Act 1, Part 2 in which Lady Bracknell and Gwendolen arrive for tea at Algernon's flat is filled with instances of situational and dramatic irony. In order to get out of going to dinner at his aunt's house, Algernon invents an excuse involving his imaginary friend Bunbury:
Algernon: It is a great bore, and, I need hardly say, a terrible disappointment to me, but the fact is I have just had a telegram to say that my poor friend Bunbury is very ill again. [Exchanges glances with Jack.] They seem to think I should be with him.
This scene is an instance of dramatic irony because, while the audience knows that Bunbury is fictional, Lady Bracknell is unaware that she is being deceived and launches into a long tirade against a man who does not exist:
Lady Bracknell: I should be much obliged if you would ask Mr. Bunbury, from me, to be kind enough not to have a relapse on Saturday, for I rely on you to arrange my music for me.
This is an absurd request, since people obviously can't decide when they experience health problems, but Lady Bracknell, who is accustomed to getting her way, doesn't seem to realize how ridiculous she sounds. Ironically, her request actually is reasonable—Bunbury does not exist—so Algernon is perfectly able to decide whether or not he has a relapse on Saturday.
An ongoing source of irony in The Importance of Being Earnest is the relationship between the name "Ernest" and its homophone, the word "earnest." Throughout the play, both Gwendolen and Cecily express their desire to marry a man named Ernest, and in Act 1, Part 2, Gwendolen explains to Jack why Ernest is the only "safe" name for a husband:
Gwendolen: We live, as I hope you know, Mr. Worthing, in an age of ideals... and my ideal has always been to love someone of the name of Ernest. There is something in that name that inspires absolute confidence.
In Act 2, Part 2, Cecily tells Algernon her reasons for wishing to marry a man named Ernest, and they are remarkably similar to Gwendolen's:
Cecily: You must not laugh at me, darling, but it has always been a girlish dream of mine to love someone whose name was Ernest. [Algernon rises, Cecily also.] There is something in that name that seems to inspire absolute confidence. I pity any poor married woman whose husband is not named Ernest.
These scenes are both instances of dramatic irony. The name Ernest "inspires absolute confidence" in both women, who seem to believe that being named "Ernest" is equivalent to actually being "earnest." The audience, on the other hand, knows that Jack created the character of Ernest as a means to escape his responsibility and live a dandy's life in the city. Regardless of whether it is Jack or Algernon taking on the role, Ernest is always a symbol of deception rather than sincerity.
Cecily's confidence in the name Ernest is especially ironic, since all she knows of her guardian's brother is that he is always getting into trouble. As an individual, Ernest is neither safe nor trustworthy, which is why Cecily is attracted to him in the first place, but she still regards his name as a sign of his good character. This proclamation is also an instance of foreshadowing—at the end of the play, Lady Bracknell forbids Algernon from being baptized and prevents him from taking on the name of Ernest, which causes Cecily to end up as the "poor married woman" she earlier pitied.
In order to express thoughts and feelings that could not be spoken aloud in polite society, many Victorians became adept at a form of clandestine communication called floriography, or the language of flowers. In floriography, different plants have specific meanings associated with them, and carefully arranged bouquets can be used to send elaborate messages.
Wilde alludes to the practice of floriography during Algernon's first encounter with Cecily in Act 2, Part 1:
Algernon: Might I have a button-hole first? I never have any appetite unless I have a button-hole first.
Cecily: A Maréchal Niel? [Picks up scissors]
Algernon: No, I'd sooner have a pink rose.
Cecily: Why? [Cuts a flower]
Algernon: Because you are like a pink rose, cousin Cecily.
Algernon and Cecily, as well-educated and wealthy Victorians, are likely familiar with the language of flowers. Cecily offers Algernon a Maréchal Niel, which is a variety of yellow climbing rose. While yellow roses are used today to symbolize friendship, during the Victorian Era they could also represent jealousy. Algernon, who desires neither a platonic nor a jealous relationship with Cecily, declines the offer and compares her to a pink rose. With this simile, he emphasizes Cecily's beauty as well as her perceived innocence. While red roses are associated with intense passion and desire, pink roses symbolize femininity, grace, and the chaste affection of a budding romance.
This simile turns out to be rather ironic. In Act 1, Part 2, Jack makes it clear that he does not trust Algernon's intentions when it comes to his ward:
Algernon: I would rather like to see Cecily.
Jack: I will take very good care that you never do. She is excessively pretty, and she is only just eighteen.
Like Jack, the audience expects that Algernon will end up seducing (or indeed, "deflowering") the young, naive Cecily. But in Act 2, Part 1, Cecily turns this assumption on its head by encouraging and even returning Algernon's flirtatious advances:
Cecily: Miss Prism says all good looks are a snare.
Algernon: They are a snare that every sensible man would like to be caught in.
Cecily: Oh! I don't think I would care to catch a sensible man. I shouldn't know what to talk to him about.
Although Cecily may not be as innocent or as delicate as she initially appears, Algernon's simile is somewhat accurate. The roses in the garden at Jack's country estate create a rustic, unspoiled atmosphere, but they are actually carefully cultivated and decorative. Similarly, Cecily's façade of girlish naiveté hides the "thornier" aspects of her personality: her obsession with her guardian's "wicked" younger brother, her stubbornness in regard to Algernon's name, and her capacity to behave in a spiteful and passive-aggressive manner toward Gwendolen.
A conversation between Cecily and Miss Prism in Act 2, Part 1 foreshadows the revelation that occurs at the end of the play:
Miss Prism: Do not speak slightingly of the three-volume novel, Cecily. I wrote one myself in earlier days.
Cecily: Did you really, Miss Prism? How wonderfully clever you are! I hope it did not end happily? I don't like novels that end happily. They depress me so much.
Miss Prism: The good ended happily, and the bad unhappily. That is what Fiction means.
Cecily: I suppose so. But it seems very unfair. And was your novel ever published?
Miss Prism: Alas! no. The manuscript unfortunately was abandoned. I use the word in the sense of lost or mislaid.
Miss Prism makes sure to specify that she did not abandon the pursuit of novel-writing—she actually physically lost the manuscript. At the time, this seems like an odd detail to emphasize, but the comment makes sense later on in the play, when Miss Prism reveals that, having confused her manuscript for a baby she was tending, she placed the former in a perambulator and the latter in a handbag.
Just as this earlier moment of foreshadowing pays off, it leads into a moment of dramatic irony. From the moment Miss Prism mentions the handbag, the audience, who learned earlier in the play that Jack was found in a handbag when he was a baby, immediately knows that he is the infant Miss Prism abandoned. In Act 3, Part 2, Jack presses Miss Prism for more information:
Jack: [Who has been listening attentively]. But where did you deposit the hand-bag?
Miss Prism: Do not ask me, Mr. Worthing.
Jack: Miss Prism, this is a matter of no small importance to me. I insist on knowing where you deposited the hand-bag that contained that infant.
Miss Prism: I left it in the cloak-room of one of the larger railway stations in London.
Jack: What railway station?
Miss Prism: [Quite crushed]. Victoria. The Brighton line.
This exchange is especially ironic because it is not enough for Jack to know that the baby in a handbag was abandoned 28 years ago at a railway station (as if multiple babies were abandoned that same year under identical circumstances), he needs to know the exact name of the railway station.
Miss Prism confirms that the handbag in which she placed the baby is the same handbag in which Jack was found, but her attitude in Act 3, Part 2, in keeping with the play's comical mood, is utterly absurd:
Miss Prism: The bag is undoubtedly mine. I am delighted to have it so unexpectedly restored to me. It has been a great inconvenience being without it all these years.
This line is ironic because Miss Prism, who for some reason seems to have gone 28 years without buying a replacement handbag, is more concerned about the bag than she is about the baby.
Miss Prism's statement that the good ended happily and the bad unhappily in her novel also foreshadows the end of the play, in which all four lovers end up happily united, with Lady Bracknell begrudgingly allowing the engagements to proceed. Cecily's comment that the ending of Miss Prism's novel seems rather unfair also foreshadows the fact that the "heroes" of the play go unpunished for all their deceit and immoral behavior.
In Act 2, Part 1, there's an instance of foreshadowing when Miss Prism and Dr. Chasuble question Jack about Ernest:
Miss Prism: More shameful debts and extravagance?
Chasuble: Still leading his life of pleasure?
From these questions, the audience can deduce that the image of Ernest that Jack has presented to the members of his household is that of an unscrupulous dandy who lives well beyond his means. This description happens to perfectly suit Algernon, who lives extravagantly and is deeply in debt. It's possible that Jack, either inadvertently or on purpose, actually based the character of Ernest on his mischievous friend.
These descriptions, in addition to Algernon's decision to disguise himself as Ernest, foreshadow the revelation in Act 3, Part 2 that Algernon has been Jack's brother all along:
Jack: Algy's elder brother! Then I have a brother after all. I knew I had a brother! I always said I had a brother! Cecily,—how could you have ever doubted that I had a brother. [Seizes hold of Algernon] Dr. Chasuble, my unfortunate brother. Miss Prism, my unfortunate brother. Gwendolen, my unfortunate brother. Algy, you young scoundrel, you will have to treat me with more respect in the future. You have never behaved to me like a brother in all your life.
Algernon: Well, not till to-day, old boy, I admit. I did my best, however, though I was out of practice.
This moment is ironic because, despite what Jack claims, Algernon has spent the entire play making fun of him, arguing with him, and needling him about his romantic life. In other words, he has behaved exactly like an irritating younger brother.
The scene in which Jack arrives at his country estate and discovers Algernon masquerading as his brother is filled with dramatic irony. The audience is already aware of Algernon's presence and is awaiting the inevitable confrontation between the two men, which makes Jack's conversation with Cecily in Act 2, Part 1 quite ironic:
Cecily: Your brother Ernest. He arrived half an hour ago.
Jack: What nonsense! I haven't got a brother.
Cecily: Oh, don't say that. However badly he may have behaved to you in the past he is still your brother. You couldn't be so heartless as to disown him.
At this point, the audience possesses information that Jack does not (that Algernon is at his house) as well as information that Cecily doesn't know (that Algernon is pretending to be Ernest). As a result, Jack's claim that he doesn't have a brother means something completely different to the audience than it does to Cecily.
The scene that follows in Act 2, Part 2 is situationally ironic—Algernon appears to be one person, but he is actually someone else—as well as dramatically ironic. For example, Jack, Algernon, and the audience are all aware that Bunbury is fictional, while Cecily is not:
Cecily: Uncle Jack, do be nice. There is some good in everyone. Ernest has just been telling me about his poor invalid friend Mr. Bunbury, whom he goes to visit so often. And surely there must be much good in one who is kind to an invalid, and leaves the pleasures of London to sit by a bed of pain.
Jack: Oh! he has been talking about Bunbury, has he?
Cecily: Yes, he has told me all about poor Mr. Bunbury, and his terrible state of health.
Cecily's comment about "the pleasures of London" is ironic because, as the audience knows, Algernon uses Bunbury as an excuse to abandon his dull social obligations and financial difficulties in town so he can lead a life of pleasure in the country. It is also ironic that Cecily views Algernon's relationship with Bunbury as evidence of his good character, since the audience knows that Bunbury is a deception and therefore evidence of the exact opposite.
In order to express thoughts and feelings that could not be spoken aloud in polite society, many Victorians became adept at a form of clandestine communication called floriography, or the language of flowers. In floriography, different plants have specific meanings associated with them, and carefully arranged bouquets can be used to send elaborate messages.
Wilde alludes to the practice of floriography during Algernon's first encounter with Cecily in Act 2, Part 1:
Algernon: Might I have a button-hole first? I never have any appetite unless I have a button-hole first.
Cecily: A Maréchal Niel? [Picks up scissors]
Algernon: No, I'd sooner have a pink rose.
Cecily: Why? [Cuts a flower]
Algernon: Because you are like a pink rose, cousin Cecily.
Algernon and Cecily, as well-educated and wealthy Victorians, are likely familiar with the language of flowers. Cecily offers Algernon a Maréchal Niel, which is a variety of yellow climbing rose. While yellow roses are used today to symbolize friendship, during the Victorian Era they could also represent jealousy. Algernon, who desires neither a platonic nor a jealous relationship with Cecily, declines the offer and compares her to a pink rose. With this simile, he emphasizes Cecily's beauty as well as her perceived innocence. While red roses are associated with intense passion and desire, pink roses symbolize femininity, grace, and the chaste affection of a budding romance.
This simile turns out to be rather ironic. In Act 1, Part 2, Jack makes it clear that he does not trust Algernon's intentions when it comes to his ward:
Algernon: I would rather like to see Cecily.
Jack: I will take very good care that you never do. She is excessively pretty, and she is only just eighteen.
Like Jack, the audience expects that Algernon will end up seducing (or indeed, "deflowering") the young, naive Cecily. But in Act 2, Part 1, Cecily turns this assumption on its head by encouraging and even returning Algernon's flirtatious advances:
Cecily: Miss Prism says all good looks are a snare.
Algernon: They are a snare that every sensible man would like to be caught in.
Cecily: Oh! I don't think I would care to catch a sensible man. I shouldn't know what to talk to him about.
Although Cecily may not be as innocent or as delicate as she initially appears, Algernon's simile is somewhat accurate. The roses in the garden at Jack's country estate create a rustic, unspoiled atmosphere, but they are actually carefully cultivated and decorative. Similarly, Cecily's façade of girlish naiveté hides the "thornier" aspects of her personality: her obsession with her guardian's "wicked" younger brother, her stubbornness in regard to Algernon's name, and her capacity to behave in a spiteful and passive-aggressive manner toward Gwendolen.
The majority of the humor in The Importance of Being Earnest stems from dramatic irony: the audience is always aware that Ernest does not exist and that Jack and Algernon are both pretending to be him, but the other characters are not. This dramatic irony is at its most palpable during the two parallel proposal scenes.
In Act 1, Part 2, after Jack has proposed to Gwendolen, she reveals that she has always wanted to marry a man named Ernest:
Jack: But you don't really mean to say that you couldn't love me if my name wasn't Ernest?
Gwendolen: But your name is Ernest.
Jack: Yes, I know it is. But supposing it was something else? Do you mean to say you couldn't love me then?
Gwendolen: [Glibly] Ah! that is clearly a metaphysical speculation, and like most metaphysical speculations has very little reference at all to the actual facts of real life, as we know them.
The audience, who can easily perceive Jack's agitation, knows that his question is not a metaphysical speculation but a genuine question. Jack's slip of the tongue later in the conversation is also insignificant to Gwendolen but quite meaningful to the audience:
Jack: Gwendolen, I must get christened at once—I mean we must get married at once.
In Act 2, Part 2, Algernon and Cecily have a very similar exchange:
Algernon: But, my dear child, do you mean to say you could not love me if I had some other name?
Cecily: But what name?
Algernon: Oh, any name you like—Algernon—for instance...
Cecily: But I don't like the name of Algernon.
As in the first proposal scene, the audience is aware that Algernon's name is not really Ernest and notices when he later misspeaks:
Algernon: I must see him at once on a most important christening—I mean on most important business.
This line is also ironic because the audience has just watched Jack make an appointment with Dr. Chasuble to be christened and knows that both men are intending to change their name to Ernest.
The scene in which Jack arrives at his country estate and discovers Algernon masquerading as his brother is filled with dramatic irony. The audience is already aware of Algernon's presence and is awaiting the inevitable confrontation between the two men, which makes Jack's conversation with Cecily in Act 2, Part 1 quite ironic:
Cecily: Your brother Ernest. He arrived half an hour ago.
Jack: What nonsense! I haven't got a brother.
Cecily: Oh, don't say that. However badly he may have behaved to you in the past he is still your brother. You couldn't be so heartless as to disown him.
At this point, the audience possesses information that Jack does not (that Algernon is at his house) as well as information that Cecily doesn't know (that Algernon is pretending to be Ernest). As a result, Jack's claim that he doesn't have a brother means something completely different to the audience than it does to Cecily.
The scene that follows in Act 2, Part 2 is situationally ironic—Algernon appears to be one person, but he is actually someone else—as well as dramatically ironic. For example, Jack, Algernon, and the audience are all aware that Bunbury is fictional, while Cecily is not:
Cecily: Uncle Jack, do be nice. There is some good in everyone. Ernest has just been telling me about his poor invalid friend Mr. Bunbury, whom he goes to visit so often. And surely there must be much good in one who is kind to an invalid, and leaves the pleasures of London to sit by a bed of pain.
Jack: Oh! he has been talking about Bunbury, has he?
Cecily: Yes, he has told me all about poor Mr. Bunbury, and his terrible state of health.
Cecily's comment about "the pleasures of London" is ironic because, as the audience knows, Algernon uses Bunbury as an excuse to abandon his dull social obligations and financial difficulties in town so he can lead a life of pleasure in the country. It is also ironic that Cecily views Algernon's relationship with Bunbury as evidence of his good character, since the audience knows that Bunbury is a deception and therefore evidence of the exact opposite.
Gwendolen and Cecily's tea party during Act 2, Part 2 is filled with dramatic irony. The audience, aware that Jack and Algernon are both pretending to be Ernest, eagerly awaits the inevitable misunderstanding that results from this deception:
Cecily: Mr. Ernest Worthing and I are engaged to be married.
Gwendolen: [Quite politely, rising]. My darling Cecily, I think there must be some slight error. Mr. Ernest Worthing is engaged to me.
The humor in this scene derives from the fact that, while Gwendolen and Cecily believe that they are both engaged to the same man, the audience knows perfectly well that they are engaged to two different people, neither of whom is named Ernest. Since the audience is in on Jack and Algernon's deception, it is ironic when the two women presume their fiancés to be innocent and begin to place the blame on each other:
Cecily: [Rising] To save my poor, innocent, trusting boy from the machinations of any other girl there are no lengths to which I would not go.
Gwendolen: From the moment I saw you I distrusted you. I felt that you were false and deceitful.
This entire scene is also ironic in a more general sense, in that there is a difference between how things appear and how they are (situational irony). The women appear to be having a friendly tea party and restrain themselves so as not to appear indecorous in front of the servants, but the atmosphere is undoubtedly hostile. All their words to each other seem polite on the surface, but are actually sarcastic and insulting.
An ongoing source of irony in The Importance of Being Earnest is the relationship between the name "Ernest" and its homophone, the word "earnest." Throughout the play, both Gwendolen and Cecily express their desire to marry a man named Ernest, and in Act 1, Part 2, Gwendolen explains to Jack why Ernest is the only "safe" name for a husband:
Gwendolen: We live, as I hope you know, Mr. Worthing, in an age of ideals... and my ideal has always been to love someone of the name of Ernest. There is something in that name that inspires absolute confidence.
In Act 2, Part 2, Cecily tells Algernon her reasons for wishing to marry a man named Ernest, and they are remarkably similar to Gwendolen's:
Cecily: You must not laugh at me, darling, but it has always been a girlish dream of mine to love someone whose name was Ernest. [Algernon rises, Cecily also.] There is something in that name that seems to inspire absolute confidence. I pity any poor married woman whose husband is not named Ernest.
These scenes are both instances of dramatic irony. The name Ernest "inspires absolute confidence" in both women, who seem to believe that being named "Ernest" is equivalent to actually being "earnest." The audience, on the other hand, knows that Jack created the character of Ernest as a means to escape his responsibility and live a dandy's life in the city. Regardless of whether it is Jack or Algernon taking on the role, Ernest is always a symbol of deception rather than sincerity.
Cecily's confidence in the name Ernest is especially ironic, since all she knows of her guardian's brother is that he is always getting into trouble. As an individual, Ernest is neither safe nor trustworthy, which is why Cecily is attracted to him in the first place, but she still regards his name as a sign of his good character. This proclamation is also an instance of foreshadowing—at the end of the play, Lady Bracknell forbids Algernon from being baptized and prevents him from taking on the name of Ernest, which causes Cecily to end up as the "poor married woman" she earlier pitied.
A conversation between Cecily and Miss Prism in Act 2, Part 1 foreshadows the revelation that occurs at the end of the play:
Miss Prism: Do not speak slightingly of the three-volume novel, Cecily. I wrote one myself in earlier days.
Cecily: Did you really, Miss Prism? How wonderfully clever you are! I hope it did not end happily? I don't like novels that end happily. They depress me so much.
Miss Prism: The good ended happily, and the bad unhappily. That is what Fiction means.
Cecily: I suppose so. But it seems very unfair. And was your novel ever published?
Miss Prism: Alas! no. The manuscript unfortunately was abandoned. I use the word in the sense of lost or mislaid.
Miss Prism makes sure to specify that she did not abandon the pursuit of novel-writing—she actually physically lost the manuscript. At the time, this seems like an odd detail to emphasize, but the comment makes sense later on in the play, when Miss Prism reveals that, having confused her manuscript for a baby she was tending, she placed the former in a perambulator and the latter in a handbag.
Just as this earlier moment of foreshadowing pays off, it leads into a moment of dramatic irony. From the moment Miss Prism mentions the handbag, the audience, who learned earlier in the play that Jack was found in a handbag when he was a baby, immediately knows that he is the infant Miss Prism abandoned. In Act 3, Part 2, Jack presses Miss Prism for more information:
Jack: [Who has been listening attentively]. But where did you deposit the hand-bag?
Miss Prism: Do not ask me, Mr. Worthing.
Jack: Miss Prism, this is a matter of no small importance to me. I insist on knowing where you deposited the hand-bag that contained that infant.
Miss Prism: I left it in the cloak-room of one of the larger railway stations in London.
Jack: What railway station?
Miss Prism: [Quite crushed]. Victoria. The Brighton line.
This exchange is especially ironic because it is not enough for Jack to know that the baby in a handbag was abandoned 28 years ago at a railway station (as if multiple babies were abandoned that same year under identical circumstances), he needs to know the exact name of the railway station.
Miss Prism confirms that the handbag in which she placed the baby is the same handbag in which Jack was found, but her attitude in Act 3, Part 2, in keeping with the play's comical mood, is utterly absurd:
Miss Prism: The bag is undoubtedly mine. I am delighted to have it so unexpectedly restored to me. It has been a great inconvenience being without it all these years.
This line is ironic because Miss Prism, who for some reason seems to have gone 28 years without buying a replacement handbag, is more concerned about the bag than she is about the baby.
Miss Prism's statement that the good ended happily and the bad unhappily in her novel also foreshadows the end of the play, in which all four lovers end up happily united, with Lady Bracknell begrudgingly allowing the engagements to proceed. Cecily's comment that the ending of Miss Prism's novel seems rather unfair also foreshadows the fact that the "heroes" of the play go unpunished for all their deceit and immoral behavior.
In Act 2, Part 1, there's an instance of foreshadowing when Miss Prism and Dr. Chasuble question Jack about Ernest:
Miss Prism: More shameful debts and extravagance?
Chasuble: Still leading his life of pleasure?
From these questions, the audience can deduce that the image of Ernest that Jack has presented to the members of his household is that of an unscrupulous dandy who lives well beyond his means. This description happens to perfectly suit Algernon, who lives extravagantly and is deeply in debt. It's possible that Jack, either inadvertently or on purpose, actually based the character of Ernest on his mischievous friend.
These descriptions, in addition to Algernon's decision to disguise himself as Ernest, foreshadow the revelation in Act 3, Part 2 that Algernon has been Jack's brother all along:
Jack: Algy's elder brother! Then I have a brother after all. I knew I had a brother! I always said I had a brother! Cecily,—how could you have ever doubted that I had a brother. [Seizes hold of Algernon] Dr. Chasuble, my unfortunate brother. Miss Prism, my unfortunate brother. Gwendolen, my unfortunate brother. Algy, you young scoundrel, you will have to treat me with more respect in the future. You have never behaved to me like a brother in all your life.
Algernon: Well, not till to-day, old boy, I admit. I did my best, however, though I was out of practice.
This moment is ironic because, despite what Jack claims, Algernon has spent the entire play making fun of him, arguing with him, and needling him about his romantic life. In other words, he has behaved exactly like an irritating younger brother.