“Mum’s the word” has nothing to do with Philadelphia’s Mummers or your dear old mum (or your dear ever-young mum, of course).
The expression recalls an old usage of the word “mum,” meaning “silent.” It’s a word that’s been used since the fourteenth century. So keeping it “mum,” or telling some “mum’s the word,” is all about silence.
My favorite old usage is when it’s said as an interjection: “mum!”
I wouldn’t recommend shouting, “Mum!” these days, though. All that will most likely happen is a bunch of moms turning their heads. And then they might shush you. Wouldn’t that be ironic?
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