In chapter 9, Atticus takes Jem and Scout to visit Aunt Alexandra, Frances her son, and Uncle Jack. Frances antagonizes Scout into a fight by first talking bad about Dill and then going further to insult Atticus. He informs Scout that the family looks down on Atticus. Frances says:
Grandma says it's bad enough [Atticus] lets you all run wild, but now he's turned out a nigger-lover we'll never be able to walk the streets of Maycomb agin. He's ruinin' the family, that's what he's doin'.
This infuriates Scout and she chases him, eventually punching him in the mouth. Uncle Jack stops Scout before she can do more damage. Upon finding out that Scout has used foul language, he proceeds to spank her. Later, Uncle Jack tries to explain to Scout why she deserved a spanking and Scout tells him that he wasn't being fair and that he doesn't "understand children much." When he allows her to explain, she tells him that he didn't give her a chance to tell her side of the story, and that "Atticus doesn't just listen to Jem's side of it, he hears [hers] too." She also said that he had told her never to use foul language "except in ex-extreme provocation, and Frances provocated [her] enough to knock his block off-"
Later, when Uncle Jack talks to Atticus about the incident, he says that Scout doesn't know the meaning to half the bad language she uses. Uncle Jack tells Atticus that he avoided answering her question when she asked what a "whore-lady" was. Atticus immediately corrects him and tells him,
When a child asks you something, answer him for goodness' sake. But don't make a production of it. Children are children, but they can spot an evasion quicker than adults, and evasion simply muddles 'em...Bad language is a stage all children go through, and it dies with time when they learn they're not attracting attention with it.
By telling Uncle Jack this, Atticus is merely letting him know that it is best to be honest because if he's not, then children won't trust him. And trust is a big part of Atticus' parenting style.