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There's an English (probably specifically British) idiom:

Mum's the word

It basically means that something is secret, and not to talk about it. It can variously mean:

  • I won't talk about this.
  • Don't talk about this.
  • Hide this thing.
  • Lie about this subject if you have to.

There are phrases derived from it, such as 'to keep mum'.

Also, 'mum' is a British shortening of 'mother'.

So, why is 'mum' synonymous with secrecy? What are the origins of the phrase?

closed as off-topic by Edwin Ashworth, Davo, Janus Bahs Jacquet, choster, NVZ Oct 11 '17 at 16:32

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    The Phrase Finder is a general reference. – Edwin Ashworth Oct 5 '17 at 13:45
  • I've never knowingly heard this idiom used with the sense of Lie about this subject if you have to. So far as I'm concerned, it always means I / you / we mustn't talk about it at all (truthfully or otherwise, and regardless of whether the subject matter is "secret"). It's entirely coincidental that mum also happens to be a BrE shortening of mother. – FumbleFingers Oct 6 '17 at 16:53
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It refers to the interjection mum meaning an inarticulate closed-mouth sound.

Mum's the word:

  • Please keep quiet; don't say anything (1704+)

(The Dictionary of American Slang)

  • "be silent," 1560s, from Middle English mum, mom (late 14c.), inarticulate closed-mouth sound, indicative of unwillingness or inability to speak.

  • As an adjective meaning "secret" from 1520s. Phrase mum's the word is first recorded 1704.

(Etymonline)

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    And these expressions long pre-date the use of 'Mum'; as a pet name for one's mother, though I believe there was a WWII slogan 'Be like Dad, keep Mum' which punned on it. – Kate Bunting Oct 6 '17 at 8:06

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