Sixpence. Simple Simon: a natural, a silly fellow; Simon Suck-egg, sold his wife for an addle duck-egg.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1736 dictionary of canting and thieving slang.
Simkin * To SimperNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes