Meat between veal and beef, the flesh of an old calf; a military simile, drawn from the officer of that denomination, who has only the pay of a lieutenant, with the rank of captain; and so is not entirely one or the other, but between both.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Captain Copperthorne’s Crew * Captain PoddNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes