keel (n.)
"lowest timber of a ship or boat," mid-14c., probably from a Scandinavian source (compare 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. Keel still is used locally in England and U.S. for "flat-bottomed boat," especially on the Tyne.
keel (v.2)
"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.
keel (v.1)
1838, American English, from keel (n.). To keel over (1876) is from the nautical image of a ship turning keel-up. Related: Keeled; keeling.