Where Did the Saying “Bolt From the Blue” For a Surprise Come From and What Does the Expression Mean?

The word bolt has many uses, but all suggest surprising quickness and all originated as a reference to an arrow from a crossbow.

The word “thunderbolt” for lightning first appeared in the sixteenth century.

While the word “blue” as a description of a clear sky appeared about a hundred years later.

Since nothing could be more surprising than lightning from a cloudless sky, a “bolt from the blue” entered the language as a description of a sudden and unexpected event.