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"lowest timber of a ship or boat," mid-14c., probably from a Scandinavian source, cf. Old Norse kjölr "keel," Danish kjøl, Swedish köl, from Proto-Germanic *keluz, of uncertain origin. Some etymologists say this is unconnected with the keel that means "a ship, barge," which also is the root of Middle Dutch kiel "ship," Old English ceol "ship's prow," Old High German kiel, German Kiel "ship," but the two words have influenced each other. Barnhart, however, calls them cognates. This other word is said to be from Proto-Germanic *keula, from PIE *geul- "rounded vessel." Keel still is used locally in England and U.S. for "flat-bottomed boat," especially on the Tyne.
1838, American English, from keel (n.). To keel over (1876) is from the nautical image of a ship turning keel-up. Related: Keeled; keeling.
"to keep cool," from Middle English kelen, from Old English celan "to cool," from col "cool" (see cool). The form kele (from Old English colian) was used by Shakespeare, but it later was assimilated with the adjective form into the modern verb cool. Cognate with Dutch koelen, Old High German chuolen, German kühlen.
To fall down; collapse: He was so tired he was about to keel over
[1876+; fr nautical careening of a ship so that the keel is raised]