mum's the word

From mum (“silence”). Recorded since 1704.

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mum's the word

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?

   • 1879, Anna Katharine Green, The Leavenworth Case: A Lawyer's Story, G. P. Putnam's Sons, page 256:

     … and that mum is the word till I give you liberty to speak

• breathe a word
• under one's hat