In To Kill a Mockingbird, what does it mean when Uncle Jack Finch says "...the best defense to her [Miss Maudie] was spirited offense?"
Uncle Jack and Miss Maudie had known each other since they were children. They had grown up together in Finch's Landing and had developed a kind of teasing, playful relationship. Every Christmas, when Uncle Jack came to visit, he would yell out to Miss Maudie and ask her to marry him. HE made the first contact, that put him on the offensive. She would respond with
"Call a little louder, Jack Finch, and they'll hear you at the post office. I haven't heard you yet." (pg 44)
It was a playful banter between the two of them. Uncle Jack had no intention of marrying Miss Maudie, and Miss Maudie had no intention of accepting his proposal. He knew that. He just needed to get the teasing started on his terms. He told the children,
"... he was just trying to get Miss Maudie's goat, that he had been trying unsuccessfully for forty years, that he was the last person in the world that Miss Maudie would think about marrying but the first person she thought about teasing...." (pg 44)
When teasing someone, it is always best if you start it instead of respond to someone else's teasing. So, his best defense to her teasing was if he waged a "spirited offense" and teased her first.
Jack Finch, Scout and Jem's uncle, teases Miss Maudie, who he has known since they were children, by asking her to marry him each Christmas. When he yells across to her, she responds by yelling back, "Call a little louder, Jack Finch, and they’ll hear you at the post office, I haven’t heard you yet!”
Uncle Jack and Miss Maudie are simply trying to tease each other. Miss Maudie, Jack says, has absolutely no desire to marry Uncle Jack, so she yells loudly as a way to get back at him and embarrass him. The quote in the question means that Miss Maudie thinks that the best way to defend herself against Uncle Jack's jokes is to make him even more embarrassed than she is and to mount an offensive against him, which consists of her own jokes aimed at him.