One that slides into a house in the dark of the evening, and hides himself, in order to let some of the gang in at night to rob it.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Darkmans * DartNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes