A severe naval punishment during the 15th and 16th centuries. The victim, presumably a delinquent sailor, was dragged from one side of the boat to the other, under the bottom of the boat (keel). Tossed over one side and pulled up on the other, he was usually allowed to catch his breath before suddenly being tossed overboard again. Keel hauling lost favour at the beginning of the 18th century, to be replaced by the cat-o-nine-tails.
The term still means a rough reprimand.