Mum’s the Word

“You won’t tell anyone, will you?”
“Of course not. Mum’s the word.”

Have you, like me, ever wondered where this strange little expression comes from? For most of us Brits, “mum” is what we call our mothers, so is this idiom related to the fact that mothers are supposed to be good at keeping secrets? Some of them might be, but this phrase, along with its relative “to keep mum”, comes from a different, now obsolete meaning of mum.

Once upon a time, “mum” meant “silence” or “refusal to speak”, and the first written usage of it with this meaning dates from the mid-1500s, as do the idioms “mum’s the word” and “to keep mum”, which are still with us today. So although we’ve lost “mum” as a noun by itself, it still speaks through the phrases it’s part of. But shh. Don’t tell anybody!