|A WWII poster using the phrase|
“Seal up your lips, and give no words but mum.” - Henry VI (1:2)
A friend used the expression “mum’s the word” and it made me wonder about its origin. I guessed that "mum" might be some British reference to mother, although the connection to being quiet was not there, other than -mother telling you to be quiet. In its usage, the phrase always seems to have a secretive association.
The expression dates from about 1700, but mum, which means “silence,” is much older. That goes back to around 1350 and the Middle English word momme for silence.
It might also be derived from the "mummer," a person who does pantomime and acts without saying anything.
There is a phonetically similar German word "stumm" (Old High German "stum", Latin "mutus") meaning "silent, mute".
We use the phrase as a request or warning to say nothing, often to not reveal a secret.