mum's the word

慣用表現

(idiomatic) Indicates that something is a secret and not to be divulged.

日本語の意味

秘密を厳守する、口外しない

Mum's the word about the surprise party for Jenny — keep it to yourself until the big day.

ジェニーのサプライズパーティーについては内緒にして、当日までは誰にも言わないでください。

意味（1）

(idiomatic) Indicates that something is a secret and not to be divulged.

She doesn't know anything about it yet, so mum's the word, OK?