open (one's) mouth

open (one's) mouth

To say something, especially that which is indiscreet, boorish, or meant to be a secret. Why did you have to open your mouth about my crush on Tommy? Why couldn't you just keep it between us? You've got to make sure that he never opens his mouth about what went on here today.
See also: mouth, open
Farlex Dictionary of Idioms.

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:
  • say over
  • be on the market
  • why, oh why
  • why doesn't (one) (do something)
  • why don't you?
  • why keep a dog and bark yourself
  • Why keep a dog and bark yourself?
  • It's anybody's guess
  • it's anybody's/anyone's guess
  • be anybody's/anyone's guess