Mum’s the word

This expression dates from the 15th century and is an injunction to remain silent and not say a word. The word mum here has no relevance to mother. Mum is an old word from the 1400s, imitative in origin, and means the sound one makes with closed lips. By the 1500s, it meant refusing to speak or to stay silent, from which the word mummer, an actor in a dumb show, is derived. Shakespeare uses a variation of the expression in Henry VI Part II, Act I, Scene II, “Seal your lips and give no word but mum.”