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.
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.