mum's the word!

mum's the word

A request or pledge to keep something a secret. Now, I don't want anyone to know that I'm pregnant yet, so mum's the word. Your secret is safe with me. Mum's the word.
See also: word
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

ˌmum’s the ˈword!

(informal) used for telling somebody to keep a secret or for telling somebody that you will keep a secret: ‘Nobody must mention this project outside the office. I hope that’s clear.’ ‘We understand, John. Mum’s the word!’
These two idioms refer to the sound you make when your mouth is closed.
Farlex Partner Idioms Dictionary © Farlex 2017
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References in periodicals archive ?
MUM'S the word! Twycross Zoo, near Tamworth, is urging sons and daughters to give their mums a gift this Mother's Day - a visit to see the new babies with their mothers.
Mum's the word! It's natural to overeat for a few months after a period of starvation.