1806, "backsight of a firearm," from hind (adj.) + sight (n.). Meaning "a seeing what has happened, a seeing after the event what ought to have been done" is attested by 1862, American English, (in proverbial "If our foresight was as good as our hindsight, it would be an easy matter to get rich"), probably formed as a humorous opposition to older foresight (q.v.).
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