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.