not open 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. © 2015 Farlex, Inc, all rights reserved.

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. © 2002 by The McGraw-Hill Companies, Inc.
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