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
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.