Scout unintentionally causes everyone to laugh when she replies to the question with, "Under my dress" (229). Actually, it is Miss Maudie who asks the question of Scout in Chapter 24 of To Kill a Mockingbird. It may help to see the comment within its context:
Miss Maudie's gold bridgework twinkled. "You're mighty dressed up, Miss Jean Louise," she said. "Where are your britches today?"
"Under my dress."
I hadn't meant to be funny, but the ladies laughed. My cheeks grew hot as I realized my mistake, but Miss Maudie looked gravely down at me. She never laughed at me unless I meant to be funny. (229)
The basis of the joke revolves around the term "britches" which can mean "jeans" or "pants" or "underwear." Miss Maudie, of course, is surprised to see Scout wearing a dress instead of her usual "britches" or pants that she usually wears (being a tomboy and all). This is the basis for Miss Maudie's comment. Scout, of course, talks without thinking here, . . . assuming that Miss Maudie is referring to her underwear and forgetting that she is in the company of the high class ladies of Maycomb. Miss Maudie's modest response shows what great respect she has for the Finch children.