Green’s Dictionary of Slang

a refusal to speak, silence; the state of being dumb; latterly in phr. mum’s the word under mum adj.

1533		J. Heywood Play of Love in Farmer Dramatic Writings (1905) 180: Then wist I well the noddy must come / To do as he did, or stand and play mum.
1546		J. Heywood Proverbs II Ch. v: I will saye nought but mum, and mum is counsell.
1555		J. Heywood Epigrams upon Proverbs (3rd hundred) xvii: Mum is counsel in every man we see / But mum except, nothynge is councell in thee.
1568		U. Fulwell Like Will to Like 11: Well, godfather, no more words but mum!
1576		Tyde taryeth no Man in Collier (1863) II 16: But Husht, syrs! I say: no moe wordes but mum.
c.1590		Jeronimo (1605) D: Peace no words Ile get thy pardon. Why, mum, then.
1613		Middleton Chaste Maid in Cheapside I i: sir walter: Davy, not a word. davy: Mum, mum, sir.
1641		R. Brome Jovial Crew V i: sen.: Not a word to any body. ra.: Mum.
1660		J. Tatham Rump I i: Oddso they are here, I cry Mum —.
1672		‘The Bathing Girles’ in Ebsworth Westminster Drolleries (1875) II 103: Every Lass her man did pray, That what had past, no more of that but Mum.
1690		A Weeks Loving, Wooing and Wedding [song] He said his Love was on her set, / but she said nought but mum to’t.
c.1698		B.E. Dict. Canting Crew n.p.: Mum-for-that, not a Word of the Pudding.
1700		Congreve Way of the World V ii: S’heart an she should, her forehead would wrinkle like the coat of a cream-cheese; but mum for that, fellow-traveller.
1719		in D’Urfey Pills to Purge Melancholy III 276: To the Dr. then he went, / For to give himself Content, / And to cure his Wife of the mum, mum, mum.
1722		A. Ramsay ‘The Caterpillar and the Ant’ Fables and Tales 19: The Caterpillar was struck dumb / And never answer’d her a Mum.
c.1770		‘Whose Three Hogs Are These?’ in Farmer Merry Songs and Ballads (1897) III 278: But as for John Cook’s Wife, I’ll say no more than mum.
c.1780		‘The Sailor’s Frolick’ in Holloway & Black (1975) I 238: I rose up in a rage, and little said but mum.
1797		‘T.B. Junr’ Pettyfogger Dramatized II ii: ‘Mum should be the Order of the day,’ before a Gentleman of the Law.
1816		W.T. Moncrieff All at Coventry I i: They’ve charged for two as I used to do for a double tooth on Mutton Hill; but mum for that though.
1830		Egan Finish to the Adventures of Tom and Jerry (1889) 46: mum became the order of the day between him and the Old Maid.
1846		Durivage & Burnham Stray Subjects (1848) 87: It’s all ‘mum,’ you know – nothing to be said, eh?
1885		Bulletin (Sydney) 18 July 12/3: There was a fellow […] who made quite a good thing of the business, […] undertaking to refrain from reporting certain cases for a consideration. […] On this the ‘purveyor of mum’ was arrested, but […] let loose again next day.