Keep your mouth shut
Also found in: Acronyms.
keep (one's) mouth shut
To not discuss or mention something. He needs to keep his mouth shut and stop telling our family secrets to everyone in town!
Keep your mouth shut (about someone or something).
Fig. Do not tell anyone about someone or something. Bob: Are you going to see the doctor? Mary: Yes, but keep your mouth shut about it. Bob: Isn't Tom's uncle in tax trouble? Jane: Yes, but keep your mouth shut about him.