The idea of people as "trash" is discussed throughout the novel. Develop a definition of what it is to be "trash," and explain your definition...

"White trash" is a slang term that evolved in the South for the stereotypical character who is poor, lazy, ignorant, shiftless, of low intelligence and usually dirty and often inbred. Even when presented with opportunities to improve their conditions, few take advantage of them as they are unmotivated.  If they do try to improve in some way, they often regress and return to their habitual conditions. Ethical behavior is usually lacking in such people, as well.

One novel that contains the quintessential "white trash" is Erskine Caldwell's Tobacco Road. In this novel about a Georgia sharecropper, whose land is played out, the daughters leave for Augusta, which is nearby in order to work in factories and to get away from their incestual father. But, he remains, although he is starving because he loves his land and remembers better days. When he does have an opportunity to make some money with the thirty-nine-year old woman that marries his sixteen year-old son, he and the others mishandle themselves and everything else and get nowhere; in fact, they make their lives worse. 

Similarly, in To Kill a Mockingbird the Ewells are a family in which poverty, drunkenness, incest, indolence, meanness, laziness, and uncleanliness are pronounced. The father, Bob Ewell, is a shiftless man who squanders his welfare check on alcohol, and the children hunt through garbage at the dump for sustenance; they do not try to do chores for anyone for food or pay, either.
Not only is Bob Ewell ignorant, but he is disagreeable and odious in his neglect for his children as well as his cruelty to them, and his disrespect for the law and attitude toward African-Americans, the only people over whom he can feel superior. Moreover, because blacks are the only ones over whom he can feel better, Ewell accuses the kind Tom Robinson of raping and beating Mayella when he himself has beaten her severely after finding that she has invited Tom inside their shack.

Both he and his children have hateful attitudes toward anyone in authority, as, perhaps, a defense mechanism. While he and Mayella both are on the witness stand, they are impolite and confrontational toward Atticus and Mr. Gilmer both. When, for instance, Mr. Gilmer asks Ewell if he is the father of Mayella, he answers impertinently, "Well, if I ain't I can't do nothing about it now, her ma's dead," and Judge Taylor interrupts to correct Ewell on his tone. Throughout his testimony, Ewell is hostile, using inappropriate language and he displays his ignorance when he does not know the meaning of ambidextrous.
When Mayella is on the witness stand, she lies about Tom's actions, from dread of her father, as well as fear from society as she has broken an unwritten law in the South. Instead of showing any shame, she tries to disguise her guilt by antipathy toward Atticus, which is not even reasonable, as she tells Judge Taylor,

"Long's he keeps on callin' me ma'am an sain' Miss Mayella. I don't hafta take his sass, I ain't called upon to take it."

Much like his father, Ewell's son, Buress, who is in Scout's class at school is filthy and lice ridden, yet he presumes to speak with utter disrespect to Miss Caroline: "I done done my time for this year." When Miss Caroline tells Burris to remain and sit down, he challenges her, "You try and make me, missus."