Uncle Jack likes to joke around with Miss Maudie. This is understandable, given that she has such a lively sense of humor. Every Christmas when he comes over to visit his brother and the kids, he always yells across the street at Miss Maudie, jokingly requesting her to come and marry him. She, in turn, always responds by hollering, "Call a little louder, Jack Finch, and they’ll hear you at the post office, I haven’t heard you yet!"
Miss Maudie and Uncle Jack have known each other since they were kids, and for the last forty years, Jack's been trying manfully to get Maudie's goat, but without success. Jack's the last person on earth that Maudie would ever consent to marry, but he's the very first person that she'd consider teasing. So Jack figures that it's best to get his retaliation in first. And as attack is often the best form of defense, he always makes sure that he's the one to start teasing. This is what's meant by "spirited offense."