open your 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

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. © 2015 Farlex, Inc, all rights reserved.

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 © Farlex 2017
See also:
References in classic literature ?
"Open your mouth, sir, and take a sup of this." My mother was rejoicing over me on one side of the bed; and the unknown gentleman, addressed as "doctor," was offering me a spoonful of whisky-and-water on the other.
"You will understand, that every time you open your mouth, it will be to take a sup of this, and not to speak.
Los Angeles, CA, September 25, 2018 --( Riding high on the success of his new hit single "Open Your Mouth and Say Something", Brent Jones' new CD& Songbook hits a stunning 12 Billboard charts including Billboard's prestigious Top 200 Albums chart.
The open your mouth picture is quite incredible - children open their mouth wide open in an unexpected way.
"He has to learn that you don't open your mouth when you get into situations because you end up sitting in the stand."
Lance Barrett from Texas said: "Better to keep your mouth shut and have people wonder if you are a fool than to open your mouth and remove all doubt.
"Smilodon could open its mouth wider than any modern cat," explained Dr Christiansen, "because if you have big teeth, you have to open your mouth at a very high angle to get anything in your mouth."
Meaning you have to open your mouth under the beer tap.
A: If you could open your mouth as wide as a snake can, you could swallow a watermelon whole.
My friend said close your eyes, open your mouth and it's a done deal.
When confronting a colleague who's not pulling his or her own weight, don't open your mouth until you've opened your mind.
WE were just about to go for a take and it was a kissing scene and I said Johnny open your mouth and he opened his mouth and I squirted gold spot in it because he had garlic the night before.
We mentioned an important difference between presentations to technical peers and those to non-technical audiences: when you talk to peers, you can often count on an atmosphere of common challenge or shared commitment, even before you open your mouth. With non-technical audiences, you have to create this shared commitment.
For instance, Pagels tells of Irenaeus's opposition to a self-styled prophet named Marcus, who at his services would place his hands on each initiate's head and say, "Behold, grace has descended upon you; open your mouth, and prophesy." According to Irenaeus, the initiate would protest that he or she is not a prophet and does not know how to prophesy.