What is "mum's the word"

Mum \Mum\, a. [Of imitative origin. Cf. Mumble.] Silent; not speaking; as, to keep mum.
--Thackeray.

The citizens are mum, and speak not a word.
--Shak.

mum's the word keep this a secret; don't tell anybody.

phr. (context idiomatic English) (non-gloss definition: The accompanying facts are a secret, not to be divulged.)

Mum's the word is a popular English idiom. It is related to an expression used by William Shakespeare, in Henry VI, Part 2.