A society of gentlemen, eminent for wit and learning, who in the reign of queen Anne and George I. met at a house kept by one Christopher Cat. The portraits of most of the members of this society were painted by Sir Godfrey Kneller, of one size; thence still called the kit-cat size.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Kit * Kitchen PhysicNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes