means The loud rumbling or crashing noise that follows a flash of lightning, caused by the rapid expansion of air heated by the bolt.

from From Old English 'thunor,' the same word that gave us the god Thunor (Norse Thor), whose hammer-blows were said to make the sky boom. It traces back to Proto-Germanic 'thunraz' and is a cousin of Latin 'tonare,' to thunder — all echoing a very old root meaning simply 'to resound.' The 'd' snuck into the spelling later, a common English habit of slipping consonants between 'n' and 'r'.