mum1

 
Pronunciation: /mʌm/

adjective

[predic.]
  • silent.

Phrases

keep mum

informal remain silent, especially so as not to reveal a secret: he was keeping mum about a possible move to West Ham

mum's the word

informal (as a request or warning) say nothing; do not reveal a secret: I know I can rely upon your discretion, mum’s the word

Origin:

late Middle English: imitative of a sound made with closed lips