early 14c., opinioun, "a judgment formed or a conclusion reached, especially one based on evidence that does not produce knowledge or certainty," 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- (2) "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"]
The word always has tended toward "a judgment or view regarded as influenced more by sentiment or feeling than reason." The meaning "formal statement by a judge or other professional" is from late 15c. The specific sense of "the estimate one forms of the character or qualities of persons or things" is by c. 1500. Public opinion, "the prevailing view in a given community on any matter of general interest or concern," is by 1735.
Middle English, perhaps reflecting the era's concern for obtaining knowledge through learned disputation, had opinional "characterized by likelihood rather than certainty" and opinial "based on probable but not certain evidence" (both mid-15c.).