washsomeone's mouth out (with soap)

rinse someone's mouth out† (with soap) & washsomeone'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.