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retroactive (adj.)[retroactive 词源字典]: 1610s, from French rétroactif (16c.) "casting or relating back," from Latin retroact-, past participle stem of retroagere "drive or turn back," from retro- "back" (see retro-) + agere "to drive, set in motion" (see act (v.)). Related: Retroactively.[retroactive etymology, retroactive origin, 英语词源]