A wooden leg: allusion to a jury mast, which is a temporary substitute for a mast carried away by a storm, or any other accident. Sea phrase.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Juniper Lecture * Jury MastNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes