keep one's mouth shut

Idioms  

1. Be quiet; don't reveal confidential information. For example, The teachers told us to keep our mouths shut during the entire presentation, or You can't tell Carol anything; she's incapable of keeping her mouth shut. Also see hold one's tongue.

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Example Sentences

It's perhaps wiser to keep one's mouth shut if you don't know the words, as former Welsh Secretary John Redwood discovered in 1993 as he struggled to mime along to Hen Wlad Fy Nhadau.

From BBC

Moreover, in life I have found it the best policy to keep one's mouth shut.

From Project Gutenberg

"Perhaps it is as well that the laws of politeness keep one's mouth shut at times, and that we do not say what we feel."

From Project Gutenberg