In Harper Lee's To Kill a Mockingbird, Atticus teaches Scout that using the word nigger is offensive and beneath her dignity.
In Chapter 9, Scout begins being persecuted by kids at school for her father's decision to defend Tom Robinson. When Cecil Jacobs makes Scout feel insulted by saying her "daddy defend[s] niggers," Scout attacks him even though she's not really sure what he means. She asks Jem what Cecil means the first chance she gets, who tells her to ask Atticus, leading Scout to next ask, "Do you defend niggers, Atticus?" Atticus replies that he certainly does but further says, "Don't say nigger, Scout. That's common."
One definition of common refers to anything that is "of mediocre or inferior quality; mean; low," meaning anything that is base, inferior, offensive, or tasteless (Random House Dictionary, 2016). Therefore, in telling Scout the word nigger is common, Atticus is saying that using the word is beneath her. He is saying that the rest of society uses the word because they are inferior people who want to be offensive by treating African Americans as inferior. In contrast, through his actions, Atticus teaches his children that all people, though not born with equal advantages and opportunities, deserve to be treated with equal respect. He treats Calpurnia with respect by calling her a "faithful member of this family" (Ch. 14), and he treats Robinson with respect by putting his all into defending Robinson in trial.
Atticus wants his children to rise above the "common" behavior displayed by many of Maycomb's citizens of the 1930s. He employs Calpurnia and gives her responsibilities and trust that most others would not have provided a black housekeeper of the time. He also explains that a white person who takes advantage of a black man is
"ten times worse than cheatin' a white man... Says it's the worst thing you can do."
Atticus tells Jem that any white man who deliberately cheats a black man "is trash." Atticus also explains that "I couldn't hold my head up in town" if he did not seek justice for Tom. Atticus tries to teach his children by his own example, and Atticus treats all people equally. It is clear that he is respected by the black community when they stand in unison following the trial, and Atticus hopes that his children will grow up to respect people of all colors in a like manner.
When Scout uses the word "nigger," Atticus intially tells her not to use the word because it is "common." What he means is that the words is crass, or slang, sonething said by "common" people, in other words, trashy. Atticus is always honest with Scout, and he confirms that he is defending an African-American in court, but also discourages her from using the word again.