One who boasts without reason, or, as the canters say, pisses more than he drinks.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1721 dictionary of canting and thieving slang.
vagaries * valentineNathan Bailey's 1721 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes