means The keel is the long structural beam running along the bottom of a boat or ship, forming its backbone and giving it stability against rolling and capsizing.

from From Old Norse 'kjǫlr,' meaning the keel of a ship — fittingly a seafaring word handed down by Norse sailors who knew their way across rough water. It entered English through Scandinavian influence and stayed put. (There's a separate, unrelated word 'keel' meaning a flat-bottomed barge, from Middle Dutch 'kiel,' which can confuse things — but the ship's-backbone sense traces to that sturdy Norse root.)