means A storm marked by thunder and lightning, usually with heavy rain, gusty wind, and sometimes hail, produced by towering cumulonimbus clouds.

from A plain compound of Old English roots: 'thunor,' the noise itself (and the namesake of Thunor/Thor, the hammer-swinging god the Anglo-Saxons heard in the sky), joined to 'storm,' a word for violent weather that runs deep through the Germanic languages. 'Thunder' is a cousin of German 'Donner' and Latin 'tonare,' to thunder — all echoing the same rumble. So the word is, quite literally, the storm that thunders.