Having a false erection. That old fellow thought he had an erection, but his—was only piss-proud; said of any old fellow who marries a young wife.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Piss Prophet * Pissing Down Any One’s BackNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes