'backfire' originates from English as a compound of 'back' + 'fire', where 'back' meant 'toward the rear' and 'fire' meant 'to set fire or discharge'.

'backfire' first appeared in English in the sense of a gun or engine firing backward or producing a backward explosion; the figurative sense 'to have an opposite or adverse effect' developed later.

Initially it meant 'to fire backward or to cause a reverse explosion,' but over time it evolved into the current figurative meaning 'to produce an effect opposite to that intended'.