A popular tune to a song, having for the burden, “Her black joke and belly so white:” figuratively the black joke signifies the monosyllable. See Monosyllable.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Black Jack-1 * Black IndiesNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes