Atticus and his sister have completely different definitions of trash. In chapter 23, Atticus and Jem are discussing the unfortunate outcome of the Tom Robinson trial, and Atticus comments on racial injustice. Atticus then tells his son,
. . . 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 (Lee, 224).
According to Atticus, a person with no integrity or respect for an individual of another race is trash. Atticus's definition of "trash" focuses on a person's character and treatment of others. Atticus dismisses a person's family background and places the emphasis on how they treat others.
Alexandra's definition of "trash" is completely different and is illustrated in her response to Scout, who asks if Walter Jr. can come over to play. Alexandra reveals her prejudice against lower-class, poor white families by telling Scout that no matter how many times someone scrubs Walter Jr. he will never be like Jem, which she says before she comments on the drinking streak in Walter's family. Scout continues to protest and ask why Aunt Alexandra will not allow her to invite Walter Jr. over to play. Alexandra finally responds to Scout by saying,
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 (Lee, 228).
According to Aunt Alexandra, a person who occupies a lower social class is considered "trash." Alexandra's definition only concerns a person's social status, family background, and appearance. She does not take into consideration how a person behaves or treats other people.