To punish someone for using rude, vulgar, or obscene language by cleaning their mouth with soap (usually used merely as a threat). If you kids don't stop cussing in this house, I'm going to wash your mouths out with soap!

An admonishment for using profanity. Used especially by parents (or other adults) to children. Used (figuatively) as an imperative. Wash your mouth out! What would your mother think if she heard you using that language!