There's an English (probably specifically British) idiom:
Mum's the word
It basically means that something is secret, and not to talk about it. It can variously mean:
- I won't talk about this.
- Don't talk about this.
- Hide this thing.
- Lie about this subject if you have to.
There are phrases derived from it, such as 'to keep mum'.
Also, 'mum' is a British shortening of 'mother'.
So, why is 'mum' synonymous with secrecy? What are the origins of the phrase?