means A violently rotating column of air that touches both the ground and a thunderstorm cloud, capable of flinging cars, houses, and debris with terrifying force.

from From Spanish 'tronada,' meaning thunderstorm (from 'tronar,' to thunder), likely tangled up with the Spanish verb 'tornar,' to turn or twist — the spinning sense and the thundering sense blurred together in sailors' mouths until the word came to mean the whirling storm itself.