This quotation is spoken by Atticus' brother, Jack, during the Finch family's Christmas gathering in Chapter 9 of To Kill a Mockingbird. Scout had earlier gotten into a fight with her cousin, Francis, in which she "split my knuckle to the bone on his front teeth." Uncle Jack had spanked Scout, blaming her entirely for the altercation. Later, when Uncle Jack took care of Scout's wounds in the bathroom after the two had made up, she asked him,
"What's a whore-lady?"
It was what Scout had called Francis earlier, but she had no idea what the term meant; to her, it was an innocent question. Jack's later statement that "Her use of bathroom invective leaves nothing to the imagination," simply means that he was astonished at the young Scout's question about such a demeaning and seemingly adult phrase. ("Invective," by the way, is defined as the use of an insulting or abusive term.) Jack avoided answering Scout's question directly, instead telling her a story about Lord Melbourne.