Part of the wick or snuff, which falling on the tallow, burns and melts it, and causing it to gutter, thus steals it away.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Thief * Thief TakersNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes