Fellows who associate with all kinds of villains, in order to betray them, when they have committed any of those crimes which entitle the persons taking them to a handsome reward, called blood money. It is the business of these thief takers to furnish subjects for a handsome execution, at the end of every sessions.
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.
Thief In a Candle * ThimbleNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes