Uncle Jack Finch says the “best defense to her [Miss Maudie] was spirited offense.” What does he mean by that?
Uncle Jack and Miss Maudie had known each other since childhood. In other words, they had a history together. They knew how to push each other's buttons and tease each other. When Uncle Jack would yell his marriage proposal across to Miss Maudie, he wasn't serious. He has told the children that Miss Maudie loved to tease him. He was,
"...the first person she thought about teasing...." (pg 44)
So, if he could get to her first and tease her or give he a bad time first, then he would be ahead of the game. In other words, if he made the first attack, hence the offense, she would have to defend herself. He liked to attack first before she had a chance.