An informer that pretends to be sleeping, and thereby overhears the conversation of thieves in night cellars.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Queer Prancer * Queer StreetNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes