Mum's the word is a popular English idiom. It is related to an expression used by William Shakespeare, in Henry VI, Part 2. Mum's the word means to keep silent or quiet. Mum is a Middle English word meaning silent. The word may be derived from the mummer who does pantomime and just acts without saying anything. Note the phonetically similar German word stumm (Old High German stum, Latin mutus) meaning silent, mute.