A dependant, poor relation, or simpleton, on whom all kinds of practical jokes are played off; and who serves as a butt for all the shafts of wit and ridicule.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Butcher’s Horse * Butter BoxNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes