mum's the word
used to tell someone that they must not tell other people about a secret

[TahlilGaran] Dictionary of Contemporary English ▲

mum's the word
do not tell anyone.
I don't want to spoil the surprise for Tim, so please tell everyone that mum's the word.

[TahlilGaran] English Idioms Dictionary ▲

Mum's the word.
[informal]
something that you say which means something should be kept secret.
I think I'm pregnant, but mum's the word until I know for sure.

[TahlilGaran] English Idioms Dictionary ▲

mum's the word
say nothing of a secret that you know
"Mum's the word on the party. I won't tell anybody."

[TahlilGaran] English Idioms Dictionary ▲