You will ride a horse foaled by an acorn, i.e. the gallows, called also the Wooden and Three-legged Mare. You will be hanged.—See Three-legged Mare.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Accounts * Act of ParliamentNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes