not open (one's) mouth

not open (one's) mouth

To be completely silent; to remain discreet (about something); to not tell anyone (about something). All the while our teacher was shouting at us, we didn't open our mouths. Don't open your mouth about this! I would die of embarrassment if anyone were to find out.
See also: mouth, not, open
Farlex Dictionary of Idioms.

not open one's mouth

 and not utter a word
Fig. not to say anything at all; not to tell something (to anyone). Don't worry, I'll keep your secret. I won't even open my mouth. Have no fear. I won't utter a word.
See also: mouth, not, open
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

not open one's mouth

Also, shut one's mouth; not say or utter a word . Be silent, repress one's feelings or opinions, keep a secret. For example, Don't worry, I'm not going to open my mouth on this issue, or She promised not to say a word about it to anyone. Also see hold one's tongue; keep one's mouth shut.
See also: mouth, not, open
The American Heritage® Dictionary of Idioms by Christine Ammer.

open your (big) ˈmouth

(informal) say something when you should not: Why do you always have to open your big mouth? Can’t you just keep quiet sometimes?
See also: mouth, open
Farlex Partner Idioms Dictionary
See also:
  • not open mouth
  • not open one's mouth
  • open your mouth
  • open the way for (one)
  • open house
  • out in the open
  • burst open
  • an open book, he/she is (like an)
  • be an open book
  • throw open