From Old Norse 'kjolr' (ship's keel) — borrowed during the Viking Age, source of 'even keel' and 'keelhaul.'
The longitudinal structural member running along the center of the bottom of a ship's hull, forming its backbone and lowest point; the foundational timber or plate from which the hull is built up.
From Old Norse 'kjǫlr' (the keel of a ship), from Proto-Germanic *keluz (keel), possibly from PIE *gel- (to be cold, to freeze — with a semantic connection through the idea of a sharp, frozen ridge, like the ridge of ice on a frozen surface). The word replaced the native Old English term and entered English during the period of heavy Norse influence in the Danelaw regions. The keel is the fundamental structural element of a ship — the first piece laid down in construction
The phrase 'on an even keel' — meaning stable and balanced — comes directly from sailing. A ship that is 'on an even keel' sits level in the water, neither listing to one side nor tilting bow-up or stern-down. The punishment of 'keelhauling' — dragging a sailor under the ship from one side to the other, scraping them along the barnacle