In Chapter 26, Mrs. Crupp (David's landlady) notices that David must be infatuated with a woman because he is preening and neglecting to focus on things that matter more than appearances. She delivers a short monologue to him in dialect, advising him to pursue things besides Dora's attention:
"[...] I should never wish to intrude where I were not welcome. But you are a young gentleman, Mr Copperfull, and my adwice to you is, to cheer up, sir, to keep a good heart, and to know your own walue. If you was to take to something, sir," said Mrs Crupp, "if you was to take to skittles, now, which is healthy, you might find it divert your mind, and do you good."
A trademark of Dickens's writing is the use of dialect to convey class status, especially lower-class status. Rather than writing standard English for these characters' dialogue and describing how they speak in dialect, Dickens always writes out their speech phonetically. Mrs. Crupp speaks in a trademark working-class dialect. For example, she pronounces "advice" as "adwice" and "Copperfield" as "Copperfull."
For all of David's desire to be a good person, he is also a social climber. He has historically been ashamed to be associated with the working class, aspiring to place himself instead among the educated elite. David's interest in Dora is not unrelated to this desire. He is charmed by her, but he is also charmed by the idea of getting married to a respectable woman. As a young Victorian man, and more importantly as the protagonist of a Bildungsroman, social success for David involves not only establishing a career and building his finances, but also supporting a nuclear family with the resources he builds.
But as the novel plays out, it becomes ever clearer that David and Dora are a bad match for one another. David needs to marry not just any woman, but the right woman. Getting his mind off Dora would in fact be a smart move for David to make. The fact that this bit of wisdom is coming from a woman who is so obviously marked by her dialect as a member of the working class indicates that the narrator (the adult version of David) has perhaps come to see what young David cannot: people with lower-class status are not necessarily to be dismissed. They can impart valuable wisdom that those of higher class status struggle to see because they are more obsessed with appearances than with personal connections.
Mrs. Crupp is far from the only example of a side character who is far richer in wisdom than in money. The Peggottys are one of the most positive influences in David's life. They can't provide him with many tangible resources, but they are enormously important to the growth of his character. They also speak in a working class dialect. In fact, Emily's schooling (which begins to affect her speech) seems to be part of what leads her to be interested in Steerforth as an educated man of higher social standing than her. While education and higher class status can provide people with more options, it can also damage people's innate wisdom.
Uriah Heep, who works his way up from a working class background, retains signs of his upbringing in his dialect. He comments on the class background that shaped his dialect in Chapter 52, when David accuses him of excessive "greed and cunning:"
"[T]here never were greed and cunning in the world yet, that did not do too much, and over-reach themselves. It is as certain as death."
"Or as certain as they used to teach at school (the same school where I picked up so much umbleness), from nine o’clock to eleven, that labor was a curse; and from eleven o’clock to one, that it was a blessing and a cheerfulness, and a dignity, and I don’t know what all, eh?’ said he with a sneer. ‘You preach, about as consistent as they did [...]"
Dickens frequently marks Uriah's working class dialect with the word "umble," used in the place of "humble." It seems likely that Uriah himself is playing up or at least trying to flag his own working class dialect by repeatedly describing his "umbleness" (rather than "humility," which would be the standard noun version of the adjective "humble"). It is important not only to Dickens but also to Uriah that everyone know he has climbed the social ladder.
In this scene, Uriah even more explicitly describes his personal history. The way he recounts his schooling suggests that for Uriah, his dialect and his attitude about class were developed together during his childhood. Whereas David's brief schooling took place at an institution that prized money, Uriah went to a charity school. He was taught in school to be "umble," that his working class status was a curse, and also that he should be grateful for his chances to work. Learning that his working class status was at once a blessing (because he could work) and a curse (because he had to work) seems to have warped Uriah's character. He does not see a way for him to win in Victorian society if he works within the system into which he was born. The way he was taught to speak about himself, by calling himself "umble," was a way of admitting his wretched position. He comes to use this descriptor stubbornly, to demonstrate his defiance against the odds stacked against him. His dialect thus flags not only his working class status, but also his bitterness toward it and his determination not to let it keep him locked out of prosperity. Uriah is undoubtedly a villain in the novel, but he is also a tragic figure because he represents the damage inequality can do to a person's integrity.