Mum's the Word

"Mum" here has nothing to do with mothers. It's an old imitation of the sound "mmm," made with the lips sealed shut, and was used in Middle English (as "momme") to mean simply "silent." The word turns up in William Langland's poem Piers Plowman in the late 1300s, and Shakespeare used a close relative of the phrase in Henry VI, Part 2: "Seal up your lips and give no words but mum." There's also a nice theatrical link: medieval Christmas "mummers," costumed performers who visited houses to act out plays or games, were traditionally required to stay silent throughout, communicating only by mime, which is very likely where "mum" picked up its association with staying deliberately quiet.