What is an example of an idiom in 'To Kill a Mockingbird chapter 21?

An example of an idiom in "To Kill a Mockingbird" chapter 21 is "cut off your nose to spite your face." This idiom means taking actions that harm oneself in order to punish or seek revenge on others. In the chapter, Atticus uses this idiom to explain why it would be foolish for Bob Ewell to challenge him.