"with the face to the rear, in the direction behind," c. 1300, from abakward, from Old English on bæc (see back (adv.), and compare aback) + -weard adjectival and adverbial suffix (see -ward). As an adverb, Old English had bæcling.
As an adjective, from 1550s. The meaning "behindhand with regard to progress" is attested from 1690s. To ring bells backward (from lowest to highest), c. 1500, was a signal of alarm for fire or invasion, or to express dismay. Another Middle English word for "backward, wrongly" was arseward (c. 1400); Old English had earsling.