1 definition by bnm15

Top Definition
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

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