mum's the word

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mum 1

Not verbalizing; silent.
Used as a command to stop speaking.
mum's the word
Say nothing of the secret you know: Mum's the word on the surprise party.

[Middle English mom, mum, perhaps imitative of closing one's lips.]

mum 2

intr.v. mummed, mum·ming, mums
1. To act or play in a pantomime.
2. To go merrymaking in a mask or disguise especially during a festival.

[Middle English mummen, from Old French momer, to masquerade, mum, probably imitative of the indistinct, muffled voices of mummers behind their masks.]

mum 3

n. Chiefly British

[Short for mummy.]

mum 4

A chrysanthemum.

mum 5

A strong beer originally brewed in Brunswick, Germany.

[German Mumme.]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
References in classic literature ?
"Her father'sh in difficultiesh, and mum's the word, my dear." To that answer he adhered, in spite of all that I could say to him.
Tickets for the Mum's the Word Ladies Charity Lunch, which runs from noon until 4pm, are priced at PS29.50 per person and include a free goody bag.
MUM'S THE WORD: Cheryl with Joan (above) and happier times with love cheat Ashley
From birth and babies to parenting trials and tribulations, MUM'S THE WORD is a hilarious look at what mums really think.
MUM'S THE WORD: Uma Thurman wants to spend more time with her children
Mum's the word during baby's first year, but more dads are taking up early parenting duties, according to research.
It's just from now on, mum's the word. Now, that's leadership.
She has also appeared in Dorothy Paul's hit play, Happy Medium, and cult stage comedy Mum's The Word.
"Mum's the Word," a program on chrysanthemums, will be presented by Eugene Garden Club member Lou Hepner and guest Frances Malpass from the Mum Society at a meeting Tuesday of the Eugene Garden Club, 1645 High St.
If you think you know whodunit, mum's the word. Perhaps this time, Sarasota could try for sophisticated indifference.
Mum's the word! It's natural to overeat for a few months after a period of starvation.