Backwards

google

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Middle English: from earlier abackward, from aback.

wiktionary

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From backward +‎ -s. See also -s(“used in the formation of certain adverbs: backwards, downwards, inwards, etc.”).

From Middle English bakwardis, bakwardis, a variant of Middle English bakwarde, bakward(“backward”). Equivalent to backward +‎ -s.

etymonline

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backwards (adv.)

1510s, from backward with adverbial genitive -s. Figurative phrase bend over backwards is recorded from 1901.