Mum’s the word

'Mum's the word' usually said in a conspiratorial tone of voice, meaning that what has just been said or revealed to you by another person is confidential and should therefore remain so. This expression dates from the 15th century and is an injunction or a promise to remain silent and not say a word to anyone. 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. The word 'mumble' derives from the same etymology. By the 1500s, 'mum' 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.”