"To Kill a Mockingbird," from Harper Lee's novel of the same name, is a metaphor that means "to hurt someone who has done no wrong." It references a comment in the novel by character Atticus Finch to his daughter Scout.
In Harper Lee's 1960 novel "To Kill a Mockingbird," Atticus Finch tells Scout that it is a sin to kill a mockingbird. Scout notes that this is the only time her father has ever told her it is a sin to do something, and she wonders why.
Scout asks family friend Miss Maudie why her father said it was a sin to kill a mockingbird. Miss Maudie answers with one of the novel's most famous quotes:
"Mockingbirds don't do one thing but make music for us to enjoy.. but sing their hearts out for us. That's why it's a sin to kill a mockingbird."
Of course, neither Atticus Finch nor Harper Lee are speaking directly about mockingbirds. Instead, they are referring to the racial injustice and cruelty that many African-Americans experienced in the 1930s, when the novel takes place. The metaphor also refers to the treatment of the novel's mentally disabled character, Boo Radley. In both examples, townspeople treat others badly even though the individuals have done nothing wrong, and this behavior is what Atticus Finch considers a sin.