Green’s Dictionary of Slang

1. a mouth; cit. 1766 is double entendre for vagina (thus cf. Madge Howlet n. (1)).

1766		‘The Butcher’ in Farmer Merry Songs and Ballads (1897) II 216: Take care that her mag with raw meat is well fed, / Lest the horns of an ox should adorn your calve’s head.
1846		Bell’s Life in Sydney 1 Aug. 2/5: Her voice assimulated to a hog's squeak, and her mag never ceased.
1850		Western Times 16 Mar. 6/1: I cum to kick again, / And to let loose my mag, / I — o good lawk! I’m in sitch pain—.
1916		T. Burke Limehouse Nights 308: Shut yer blasted mag, for Christ’s sake!

2. (UK Und.) constr. with the, confidence trickery, swindling.

1896

A. Morrison Child of the Jago (1982) 95: Those of the High Mob were the flourishing practitioners in burglary, the mag, the mace, and the broads, with an outer fringe of such dippers — such pickpockets — as could dress well, welshers and snidesmen.

3. the face.

1899

C. Rook Hooligan Nights 69: The froggy layin’ there [...] is ole mag in the sputtoon.