1550s, ternado, navigator's word for violent windy thunderstorm in the tropical Atlantic, probably a mangled borrowing from Spanish tronada "thunderstorm," from tronar "to thunder," from Latin tonare "to thunder" (see thunder (n.)). Also in 17c. spelled tornatho, tornathe, turnado; modern spelling by 1620s. Metathesis of -o- and -r- in modern spelling influenced by Spanish tornar "to twist, turn," from Latin tornare "to turn." Meaning "extremely violent whirlwind" is first found 1620s; specifically "destructive rotary funnel cloud" (especially in the U.S. Midwest) from 1849. Related: Tornadic.
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