What does 'mum's the word' mean? People say it when they won't talk about something, but why?

Answer 1

The word mum is a word meaning to keep silent.
It is an old word which is derived from the Middle English
word momme from 1350-1400.
Mum's the word is an idiom which means"Don't say anything
about it. Keep quiet". It is thought that it may derive this meaning because saying the word "mum" makes one's lips close.
My father used to say this to us when we were to surprise my mother with a special gift . He would say,"Remeber kids. Mums the word".

Answer 2

Pronounce the word mum. It closes your lips.

Answer 3

To keep quiet about something. Don't say anything.