A drum. To smack calf’s skin; to kiss the book in taking an oath. It is held by the St. Giles’s casuists, that by kissing one’s thumb instead of smacking calf’s skin, the guilt of taking a false oath is avoided.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Cake * CalvesNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes