1550s, "admit (an alien) to rights of a citizen" (implied in naturalized), from natural (adj.) in its etymological sense of "by birth" + -ize. In some instances from Middle French naturaliser. Of words or expressions, "adopt as native or vernacular," 1590s. Of plants or animals, "introduce and acclimatize in places or situations where they are not indigenous," by 1708. Related: Naturalizing.