Fell over completely (as in 'keeled over'), or having a keel (the bottom part of a ship that helps it stay balanced).
From Old Norse 'kjölr' meaning 'keel,' the bottom ridge of a ship. The verb form 'to keel over' developed as sailors' language for a ship tipping dangerously or a person collapsing.
When someone 'keels over,' they're using nautical language—they literally mean the person tips like a ship whose keel has become too exposed. This shows how sailors' vocabulary has shaped everyday English!
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