Mum's the word
Mum's the word is a popular English idiom. It is related to an expression used by William Shakespeare, in Henry VI, Part 2.[1]
The word “mum” is a slanged version of momme, which was used between 1350-1400 in Middle English with very close to the same meaning: Be silent; Do not reveal
Meaning
"Mum's the word" means to keep silent or quiet. "Mum" is a Middle English word meaning "silent". [2] The word may be derived from the "mummer" who does pantomime and just acts without saying anything.[3] Note the phonetically similar German word "stumm" (Old High German "stum", Latin "mutus") meaning "silent, mute".
Origin
The origins of the phrase can be traced back to the fourteenth century and William Langland's narrative poem, Piers Plowman:
Thou mightest beter meten the myst on Malverne hulles
Then geten a mom of heore mouth til moneye weore schewed!
It can also be seen in popular fifteenth-century Towneley Plays:[4]
Though thi lyppis be stokyn, yit myght thou say 'mum'.
The phrase notably appears in Shakespeare's Henry VI, Part 2, Act 1, Scene 2:
Seal up your lips and give no words but mum.