The word 'thunderbolt' is a transparent compound formed from two Old English elements. 'Thunder' originates from the Proto-Germanic þunraz, an onomatopoeic term imitating the deep, rumbling sound of a storm. 'Bolt' derives from Proto-Germanic bultaz, originally referring to a short, heavy arrow or projectile, a sense which evolved to metaphorically describe a sudden, darting flash of lightning. The compound, therefore, literally means "the missile or flash of thunder," perfectly capturing the phenomenon where the visible lightning strike (the bolt) is inseparably linked with the audible thunderclap. Over time, this potent imagery of sudden, explosive force was extended metaphorically to describe any startling and powerful event or piece of news.

Structure: thunder (loud noise) + bolt (lightning flash) - thunder: From Old English þunor, meaning "thunder, thunderclap," of Proto-Germanic origin, related to the sound of a loud, rumbling noise. - bolt: From Old English bolt, meaning "arrow, missile, lightning flash," of Proto-Germanic origin, here denoting a sudden, swift, and striking discharge.