Middle English: from earlier abackward, from aback
In US English, the adverb form is sometimes spelled backwards (the ladder fell backwards), but the adjective is almost always backward (a backward glance). Directional words using the suffix -ward tend to have no s ending in US English, although backwards is more common than afterwards, towards, or forwards. The s ending often (but not always) appears in the phrases backwards and forwards and bending over backwards. In British English, the spelling backwards is more common than backward.