In Chapter 3, David meets Peggotty's extended family. Peggotty uses two idioms to describe Mr. Peggotty:
[Mr. Peggotty] was but a poor man himself, said Peggotty, but as good as gold and as true as steel—those were her similes.
"Good as gold" and "true as steel" don't have immediately obvious meanings in the literal sense. "Good as gold" stems from the introduction of paper money in the two centuries prior to Dickens's novel. Many people were concerned about how the paper money could be valuable if it was simply writing on a piece of paper, and the phrase "good as gold" assuaged these concerns. It meant that if someone gave you a banknote worth 20 pounds of gold, it was the equivalent of being handed the 20 pounds of gold itself (and it was much more convenient for everyone involved). The phrase took on broader meaning as it was used more, and Peggotty uses it to describe Mr. Peggotty's good character. "True as steel," meanwhile, means he's trustworthy. Even though steel is an inanimate material that does not have human characteristics, it has long been a notoriously reliable building material. To be as true as steel is to be similarly dependable.
David is quick to point out that these idioms (which, as he writes, are also similes) come from Peggotty, not him. This insistence might indicate a hint of snobbery on David's part. Working class characters are generally the most likely to use idioms in the novel. David is keen to set himself apart not only from the working class but also from the likes of people who can't come up with their own original ways of describing people and things. After all, he prides himself on being a writer. Then again, he also prides himself on being a self-made writer and on having more sympathy for the working class than many others. By using Peggotty's idiomatic expressions, the narrator highlights his humble origins and his appreciation for the working class, even as he sets himself apart from them.
In Chapter 11, Mrs. Micawber uses a corrupted Latin phrase when she tells David about Mr. Micawber’s financial trouble and her own attempts to remain loyal and optimistic. In her usage, this phrase constitutes an idiom:
‘Mr Micawber’s difficulties are almost overwhelming just at present,’ said Mrs Micawber; ‘and whether it is possible to bring him through them, I don’t know. When I lived at home with papa and mama, I really should have hardly understood what the word meant, in the sense in which I now employ it, but experientia does it – as papa used to say.’
The Latin expression Mrs. Micawber is getting at is “experiential docet,” or “experience teaches.” She gets the phrase from her father, but in their family it has been partially mistranslated into "experientia does it." This incorrect phrase in its literal sense is somewhat puzzling and devoid of meaning. Experience does what? It is not even fully translated, so it is caught between two languages. It does not signify much of anything in either language.
Mrs. Micawber never understood this idiom when she had what she needed, and anyone rich enough for a formal education would probably not understand it either, at least in a concrete and immediate sense. Mrs. Micawber (like the formally educated) was sheltered, lacking the experience to learn what the expression meant. Ironically, "experientia does it" starts to take on figurative meaning for her when experience teaches her about financial difficulty. "Experientia," or "experience," "does it" for her, or "docet"—that is, experience is the most effective way for her to learn what her father meant with his corrupted Latin idiom. The phrase still has no literal meaning, but it represents a kind of knowledge that Mrs. Micawber has gained by growing up into working class adulthood.
Mrs. Micawber's corruption of the Latin reveals her lack of a formal education. She is learning a lot about the world even though she has not had much access to the kind of teaching wealthier people get. The corrupted expression is emblematic of the change in David’s life at this point. He thought he was going to be well-educated among the elite at Salem House. Instead, he is being educated through experience in the factory and among the working poor, who don't necessarily understand Latin but who do understand other aspects of the world.