c.1300, from abakward, from O.E. on bæc + -weard adj./adv. suffix. Backwards, with adverbial genitive, is from 1510s. Meaning "behindhand with regard to progress" is first attested 1690s. To ring bells backward (from lowest to highest), c.1500, was a signal of alarm for fire or invasion, or to express dismay.
1510s, from backward with adverbial genitive. Figurative phrase bend over backwards is recorded from 1925.