Green’s Dictionary of Slang

confidence trickery, swindling.

1823		‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc. 118: Megging ? obtaining the megs, thus: two or more fellows, pretending to be utter strangers, conspire to cheat a third by laying wagers, or otherwise do him out of his bustle.
1859		Matsell Vocabulum 53: magging Getting money by cheating countrymen with balls, patent safes, etc., etc.
1868		‘Six Years in the Prisons of England’ in Temple Bar Mag. Nov. 539: Magging is not so good a game as it used to be. It means, more particularly, swindling a greenhorn out of his cash by the mere gift of the gab.