Dick's working-class dialect is characterized by the use of certain idioms, or phrases whose significance goes beyond their literal meaning. Two of his most commonly-used idioms are "bully" and "brick." In the context of the novel, "brick" refers to a kind or decent person. When Frank gives Dick his old suit, Dick dubs him a "brick" and, when Frank doesn't understand the word, explains that he means "a jolly good fellow."
Likewise, "bully" is a positive expression of agreement or enthusiasm. For example, when Mr. Rockwell offers Dick a position as a clerk in his counting house at the end of the novel, Alger writes:
Dick was about to say “Bully,” when he recollected himself, and answered, “Very much.”
The use of these slang terms indicates Dick's lower-class status, much like his overall dialect. It's important to note that wealthier characters like Frank don't understand these terms; when Dick uses slang around these friends, the ensuing confusion emphasizes the socio-economic gaps in the relationship.
As a result, Dick learns to temper his use of slang terms, just as he discards his overall dialect for standard English. At the meeting with Mr. Rockwell, which comes at the end of the novel, Dick consciously stops himself from using the word "bully," which he knows the other man won't understand. This isn't an effort to conceal his humble origins, but rather to present himself as someone who is willing and able to adopt the mannerisms of the upper class, and is thus deserving of help and success. In this sense, moving away from dialect and working-class idioms is part of Dick's education, even though no one explicitly tells him he needs to do this.
