To mount a corporal and four; to be guilty of onanism: the thumb is the corporal, the four fingers the privates.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Corny-faced * CorporationNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes