When Uncle Jack comes over to visit the Finches for Christmas, he shows the children some pictures of his cat: Rose Aylmer. She must have a rather healthy appetite, because, as Scout observes, the cat seems to be getting rather fat. Uncle Jack replies that he should think so, given that Rose Aylmer eats all the leftover fingers and ears from the hospital. (Uncle Jack's a doctor). Jack's only joking, of course—thank goodness!—but instead of laughing Scout fires back:
Aw, that's a damn story!
By modern day standards "damn" isn't much of a cuss word, but in those days it was. It was thought especially inappropriate for young children like Scout to use such language. Uncle Jack's certainly shocked by hearing her talk like this, but his brother, Atticus, is much more laid back, as we might expect. He jokingly tells Jack that Scout's been cussing all week. But Jack still takes Scout to one side and gives her a gentle ticking-off.
But Atticus isn't all that worried. He's sure that cussing is just a phase that children go through, one which they'll eventually outgrow when it no longer attracts attention. Atticus figures that if no one kicks up a fuss over Scout's cussing, then she'll eventually stop doing it.