A punishment in use among the Dutch seamen, in which, for certain offences, the delinquent is drawn once, or oftener, under the ship’s keel: ludicrously defined, undergoing a great hard-ship.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Keel Bullies * To KeepNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes