rinse mouth out
rinse (one's) mouth out (with soap)
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 rinse your mouths out with soap!
rinse someone's mouth out (with soap)and wash someone's mouth out (with soap)
Fig. to punish one by washing one's mouth out with soap, especially for using foul language. (Usually a jocular threat.) If you say that again, I'll rinse your mouth out with soap. I will wash out your mouth if you swear.