What's the meaning of this quote from To Kill a Mockingbird?
"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.”
The previous post is a good one. I would just add that Atticus is trying to explain that it is never a proper or moral act to take advantage of someone who is (or is perceived to be) weaker. The same reasoning would apply to someone who cheats or takes advantage of a child or someone with mental deficiencies. Taking advantage of someone from a position of strength or authority without a proper legal foundation is a reprehensible act. At the time of the novel's setting in the 1930s, black citizens often had no choice but to accept a white man's terms; refusing to do so, or publicly objecting to such an agreement, often meant dangerous repercussions for the black man.
These words of Atticus Finch are said after the trial of Tom Robinson in response to Jem's questioning of the verdict against Tom:
"You just can't convict a man on evidence like that--you just can't"
Atticus explains that the men of Maycomb can and did exactly that because despite what the U.S. Constitution demands, Tom Robinson did not have a "square deal" in the courtroom. For, the jury members carried "their resentments right into a jury box." This egregious act of the jury is what prompts Atticus's words that no matter what family a white man comes from, no matter what accomplishments a man has, if he cheats a black man of his constitutional rights, that man is "trash"; that is, he is unconscionable and lacks any dignity and character. Further, he tells the children that he is sickened by "low-grade white men" who take advantage of a Negro's ignorance.