Explain Atticus' statement: "Don't pay any attention to her Jack. She's trying you out. Cal says she's been cussing fluently for a week, now."
Scout's cussing is a reflection of a developmentally normal behavior for a child her age; she is testing boundaries and learning about her own ability to control a situation. As well, Scout's cussing reveals her intense resistance to the expectations that she should act in a ladylike way, expectations that she links closely with members of her extended family like Aunt Alexandra and Uncle Jack.
Atticus is very aware of Scout's behaviors and her motivations, and because he understands her position and her personality, he does not reprimand her for cussing. He knows that, sometimes, Scout might actually relish negative attention in the form of reprimands because it proves to her that she is subverting expectations successfully. Atticus appreciates Scout's independent nature, so he leaves her to her cussing, knowing that her enthusiasm for creating a sensation amongst her audience will wane on its own once the novelty wears off and she sees that she is no longer getting a reaction from disapproving adults. His advice to Uncle Jack to just leave her be is sound, because any strong reaction from Uncle Jack will only reinforce the behavior.
Atticus says "Don't pay any attention to her Jack. She's trying you out. Cal says she's been cussing fluently for a week, now" to his brother Jack in chapter 9 after Scout says to Uncle Jack, “Aw, that’s a damn story.”
It’s interesting to note that, unlike most parents, Atticus reacts calmly to Scout’s profanity. In fact, he doesn’t even reprimand her. Atticus’ parenting style is to lead his kids to correct conclusions without being overbearing about it.
A little later in the chapter Uncle Jack does reprimand her slightly, telling her that he doesn’t want to hear any more talk like that and asking her if she wanted to grow up to be a lady. Scout, of course, does not care much about being a lady.
It’s also interesting to note the way Atticus phrases the statement, with the ironic use of the word “fluently.” People wouldn’t normally think of using cuss words in a “fluent” way. Atticus is emphasizing the opposite; that using such words actually reduces one’s ability to communicate effectively. The wry nature of the comment fits Atticus’ character perfectly.