A bad, worn-out, foundered horse; also a cowardly or faint-hearted horse-stealer.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Queer Plungers * Queer RoosterNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes