What is Atticus's and aunt Alexandra's definition of trash?
Atticus's definition of trash is a white man who takes advantage of a black man. He sees this as morally wrong, because the one in power is exploiting the one without power. If anything, the one in power should protect the one without power. Atticus makes this point in a conversation with Jem. Here is what he says:
As you grow older, you’ll see white men cheat black men every day of your life, but let me tell you something and don’t you forget it— whenever a white man does that to a black man, no matter who he is, how rich he is, or how fine a family he comes from, that white man is trash.”
A person who would fit Atticus's definition of trash would be Bob Ewell.
Aunt Alexandra's understanding of trash is quite different. Her definition does not include morality. What is more important to her is the pedigree and sophistication of a person. Therefore Aunt Alexandra does not want Scout to play with Walter Cunningham. To her, the Cunninghams are trash.
She took off her glasses and stared at me. “I’ll tell you why,” she said. “Because— he—is—trash, that’s why you can’t play with him. I’ll not have you around him, picking up his habits and learning Lord-knows-what. You’re enough of a problem to your father as it is.”
As the book progresses, Mr. Cunningham proves to be a honorable man. Atticus says that he is the only juror who is standing up for Tom Robinson. This shows that Alexandra's understanding of what trash is is flawed.
In To Kill a Mockingbird, Aunt Alexandra and Atticus define "trash" in two very different ways.
Aunt Alexandra believes in the concept of "fine folks" and the value of good breeding. She takes immense pride in the history and pedigree of the Finch family, especially since Simon Finch was the first settler in the area, which means that the Finch family can trace their lineage back farther than any other family in Maycomb. Thus, to Aunt Alexandra, "trash" is defined or embodied by the Cunningham family, a poor family in which the father is a farmer whose son Walter cannot afford lunch at school.
Atticus, on the other hand, has a much less prejudiced definition of trash. Atticus sees "trash" as white men who exploit black men, using their positions of power to take advantage of them and cheat them. The Ewells would be a good example of "trash" according to Atticus's definition.