Green’s Dictionary of Slang

a petty criminal, a cheat.

1935		Thieves Slang ms list from District Police Training Centre, Ryton-on-Dunsmore, Warwicks 6: Megsman: Card Sharper.
1938		F.D. Sharpe Sharpe of the Flying Squad 332: megsmen : North-country term for card-sharpers.
1977		D. Powis Signs of Crime 193: Megsmen Petty thieves and cheats, in a small way of business, and used contemptuously.