Old English behindan "at the back of, after," from bi "by" (see by) + hindan "from behind" (see hind (adj.)). The prepositional sense emerged in Old English. Figurative sense "not so far advanced, not on equality with" is from c. 1200. Euphemistic noun meaning "backside of a person" is from 1786. To do something behind (someone's) back "clandestinely" is from late 14c. Phrase behind the times is by 1826. Behind the scenes (1711) is from the theater; figurative sense attested by 1779.
also before-hand, "in anticipation," early 13c., from before + hand, which here is of uncertain signification, unless the original notion is payment in advance or something done before another's hand does it. Hyphenated from 18c.; one word from 19c.
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Harper, D. (n.d.). Etymology of behindhand. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/behindhand