Originally one who stole or decoyed children or apprentices from their parents or masters, to send them to the colonies; called also spiriting: but now used for all recruiting crimps for the king’s troops, or those of the East India company, and agents for indenting servants for the plantations, &c.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1736 dictionary of canting and thieving slang.
Kiddy Nippers * KidneyNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes