The long piece that runs along the bottom of a boat or ship to help it stay balanced and move smoothly through water.
From Old Norse 'kjölr' meaning the bottom of a ship. The word entered English through Viking maritime culture and has remained a nautical term for over a thousand years. It's related to the concept of the ship's backbone.
The keel is so important that in naval language, 'keeling over' originally meant turning a ship upside down as a form of punishment—it was basically the worst fate possible for a sailor!
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