An arrow, a shaft (Anglo-Saxon, bolta; Danish, bolt; Greek, ballo, to cast; Latin, pello, to drive). A door bolt is a shaft of wood or iron, which may be shot or driven forward to secure a door. A thunderbolt is an hypothetical shaft cast from the clouds; an aerolite. Cupid's bolt is Cupid's arrow.
I cannot bolt this matter to the bran, As Bradwarden and holy Austin can.
Dryden's version of the Cock and Fox.
Namque Diespiter Igni corusco nubila dividens, Plerumque, per purum tonantes Egit equos volucremque currum. ...
Horace: 1 Ode xxxiv. 5, etc.
“On Monday, Dec. 22nd , there fell a bolt from the blue. The morning papers announced that the men were out [on strike].”— Nineteenth Century, February, 1891, p. 246.
In this phrase the word “bolt” is used in the popular sense for lightning the Latin fulmen, the French foudre and tonnerre, in English sometimes for an aerolite. Of course, in strict scientific language, a flash of lightning is not a thunderbolt. Metaphorically, it means a sudden and wholly unexpected catastrophe, like a thunderbolt [flash of lightning] from a blue or serene sky.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894