In Chapter 5 of Harper Lee's To Kill a Mockingbird, narrator Scout describes Miss Maudie as responding to Scout's Uncle Jack Finch with a "spirited offense." The word offense generally refers to something that has offended or displeased someone; however, offense has also derived another meaning that is applied to sports. In sports, an offense is an attack that prevents the opposing team from scoring points. The word spirited can mean lively or even courageous. Hence, Scout is describing Miss Maudie as one who defends herself against the teases of Uncle Jack by attacking Uncle Jack in a very lively and bold manner.
In Chapter 5, Scout recounts that every year Uncle Jack travels from Nashville to Maycomb to visit the Finch family for Christmas, and every Christmas, Uncle Jack yells across the street to Miss Maudie to tell her to come and marry him. Each year, Miss Maudie reciprocates by yelling, "Call a little louder, Jack Finch, and they'll hear you at the post office, I haven't heard you yet!" (Ch. 5). Scout further recounts that, when asked, Uncle Jack explains he is just "trying to get Miss Maudie's goat," which means he is trying to irritate her. He further explains that he is the "last person in the world Miss Maudie would think about marrying but the first person she thought about teasing," and to tease someone is to laugh at the person by making jokes at the person's expense. Therefore, Miss Maudie is teasing Uncle Jack by pretending not to have heard him. Plus, she is defending herself against his teasing her through attacking him in a lively and bold way by pretending he isn't worthy of being heard. Hence, all in all, the phrase "spirited offense" simply means that Miss Maudie is attacking Uncle Jack in a lively way via her own teasing.