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1.
to keep quiet about something
I heard Elaine's parents are out of town tonight and she's lonely! I'm gonna go over. Mums the word!
by P Ha September 29, 2004
 
2.
means to keep this a secret; don't tell anybody
I don't want anybody to know about this affair until next week, so mum's the word!
by Venera December 22, 2005
 
3.
to keep quiet about something; my lips are sealed.
mum means mmmmmm (the sound made when putting your lips together)
came from Shakespeare quote:
"Seal up your lips and give no words but mum."
person 1: "Please don't tell anyone about my affair."

person 2: "Don't worry i won't. Mums the word."
by bnm15 July 04, 2009
 
4.
keep quiet about it (possibly from the sound one makes when trying to speak with closed lips, Attributed to William Shakespeare)
Remember, until further notice, mum's the word.
by The Return of Light Joker January 08, 2010