keep one's mouth shut
SAY NOTHING, keep quiet, not breathe a word, not tell a soul, keep mum, not let the cat out of the bag

[TahlilGaran] Dictionary of Contemporary English ▲

keep mouth shut
keep (your) mouth shut
[informal]
to keep something secret.
You can trust Sarah - she knows how to keep her mouth shut.

Do you think I should keep my mouth shut about seeing Jim with another woman?
(sometimes + about)

[TahlilGaran] English Idioms Dictionary ▲

keep one's mouth shut
be/stay silent
I was very angry and I told my friend to keep his mouth shut. Later I had to apologize.

I tried hard to keep my mouth shut during the boring speech.

[TahlilGaran] English Idioms Dictionary ▲

keep one's mouth shut
v. phr., informal To be or stay silent.
■ A rude expression when used as a command.
When the crooks were captured by the police, their leader warned them to keep their mouths shut.
Charles began to tell Barry how to kick the ball, and Barry said angrily, "Keep your mouth shut!"
Synonym: SHUT UP¹.

[TahlilGaran] English Idioms Dictionary ▲