8 Words That Are Older Than You Think
c.1300, from Old French opinion "opinion, view, judgements founded upon probabilities" (12c.), from Latin opinionem (nominative opinio) "opinion, conjecture, fancy, belief, what one thinks; appreciation, esteem," from stem of opinari "think, judge, suppose, opine," from PIE *op- "to choose" (see option).
Where there is much desire to learn, there of necessity will be much arguing, much writing, many opinions; for opinion in good men is but knowledge in the making. [Milton, "Areopagitica"]