Mum’s the word!
Definition 1 of 1
Pro Tip 1/3Secrecy Emphasis
Emphasizes the need for secrecy about a specific piece of information. 
You know the surprise party for Lisa? Mum’s the word until her birthday!
Pro Tip 2/3Cultural Phrase
Originally British, but understood in various English-speaking cultures. 
So you've seen the unpublished manuscript? Mum’s the word, mate!
Pro Tip 3/3Informal Setting
Used in casual or informal conversations, not in formal ones. 
I snuck a cookie from the jar—mum’s the word, okay?