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1540s, "to trifle," of unknown origin; of the fingers, first recorded 1670s. Figurative phrase twiddle one's thumbs "have nothing to do" is recorded from 1846; to twirl one's thumbs in the same sense is recorded from 1816. Related: Twiddled; twiddling.
Perfect foresight of what has already been seen: observers empowered with 20-20 hindsight wanted to know (1962+)