In Book 3, Chapter 4, Merry and Pippin meet Treebeard, the eldest of the Ents in Fangorn. The Hobbits tell Treebeard about Frodo and Sam's mission to bring the Ring to Mordor. Treebeard, speaking in Westron (or "Common Speech"), responds in his usual long-winded way, including an idiom befitting the Ents:
There is something very big going on, that I can see, and what it is maybe I shall learn in good time, or in bad time. By root and twig, but it is a strange business: up sprout a little folk that are not in the old lists, and behold! the Nine forgotten Riders reappear to hunt them, and Gandalf takes them on a great journey, and Galadriel harbours them in Caras Galadhon, and Orcs pursue them down all the leagues of Wilderland: indeed they seem to be caught up in a great storm. I hope they weather it!
Treebeard says, flustered, "By root and twig, but it is a strange business." "By root and twig" seems to be an Entish idiom, meaning that he is speaking earnestly and authentically. This idiom suggests that the Ents see their physical form, their roots and twigs, as spiritual objects against which they might take an oath. This is fitting with the Ents' conception of themselves as ancient, nearly divine beings. Furthermore, small idiomatic details like this show how the Entish language influences how Treebeard speaks Westron. This allows Tolkien to flesh out his depiction of the Ents indirectly, as Tolkien implies that the Ents have their own unique set of idioms that reflect their beliefs and character.
Visiting Ithilien in Book 4, Chapter 5, Frodo and Samwise converse with Faramir about the death of his brother, Boromir, at the beginning of Book 3. Faramir uses an idiom from his upbringing in Gondor to describe how he knew of his brother's death before the Hobbits told him:
Sam sat down heavily with a red face. Faramir turned to Frodo again. ‘You asked how do I know that the son of Denethor is dead. Tidings of death have many wings. Night oft brings news to near kindred, ’tis said. Boromir was my brother.’
Faramir recites that "Night oft brings news to near kindred," following up with "'tis said," showing that this is an idiom in Gondor dialect. The idiom claims that relatives often know of each other's actions and emotions even across distance. This shows that Gondor's people value familial connections, even as an often-warring monarchy tends to send its sons off to war. Furthermore, simply using the idiom shows Faramir's connection to his upbringing in Gondor through his speech. Note also that Faramir uses the idiom literally here: he found Boromir's horn floating down Anduin three nights after Boromir's death. The river's constant flow is what brought Faramir the news during the night. Thus Faramir uses the idiom in a complex way that shows his connection to his brother, his people, and the surrounding landscape.