'backflap' originates from English, specifically the compound of 'back' and 'flap', where 'back' meant 'rear part' and 'flap' meant 'a piece of material that hangs or moves loosely.'

'back' comes from Old English 'bæc' meaning 'back, rear'; 'flap' comes from Middle English 'flappen' (verb) meaning 'to flutter' (likely imitative). The compound form 'backflap' developed in modern English as a descriptive compound for a flap at the back of something.

Initially it referred generally to 'a loose or hanging piece at the back'; over time it came to be used specifically for items like the rear flap of a book's dust jacket or the back flap of an envelope.