tornado: [16] Tornado appears to denote etymologically something that ‘turns’, but this is due to a piece of English folk-etymologizing. Its actual source is Spanish tronada ‘thunderstorm’, a derivative of the verb tronar ‘thunder’ (which in turn went back to Latin tonāre ‘thunder’, source of English astonish, detonate, etc). It was at first used in English for a ‘violent thunderstorm’, but confusion with Spanish tornado ‘turned’ had converted tronada into tornado, and as early as the 1620s we find it being applied to a ‘whirlwind’. => thunder
tornado (n.)
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.
