His calves are gone to grass; a saying of a man with slender legs without calves. Veal will be cheap, calves fall; said of a man whose calves fall away.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Calf-skin Fiddle * Calves Head ClubNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes