means A single discharge of lightning together with its crack of thunder, especially imagined as a hurled missile striking from the sky.

from A plain compound of "thunder" and "bolt." The "bolt" part originally meant an arrow or crossbow quarrel — so a thunderbolt is literally the sky's arrow. The word goes back to Old English ("thunor" gave us Thursday, Thor's day), and the image of a god flinging a bolt is ancient: Zeus and Jupiter both wielded one, which is why we still speak of being "struck" by sudden love or sudden news.