When Dr. Finch, or Jean Louise's Uncle Jack, shows up in Part 3, Chapter 6, Lee uses a metaphor to describe his disposition:
Dr. Finch had drunk so long and so deep of his heady brew that his being was shot through with curious mannerisms and odd exclamations.
This metaphor is useful in describing Uncle Jack's character, which is quite eccentric. He is interested in Victorian literature, even though he has been a bone doctor his entire life, and he can be difficult to understand to some people because his speech is peppered with Victorianisms.
One can imagine a drunk person in the street who is friendly and talkative but also incomprehensible; Uncle Jack speaks a language that not many can parse.
However, this sloppiness is just a facade:
He gave strangers the idea that he was a borderline case, but those who were tuned to his wavelength knew Dr. Finch to be of a mind so sound, especially when it came to market manipulation, that his friends often risked lengthy lectures on the poetry of Mackworth Praed to seek his advice.
If one gets past his initial ramblings on rather obscure poets, they can get truly helpful advice, which Jean Louise will seek out several times throughout the novel.