"obstinately attached to one's opinion," 1590s, past-participle adjective from opiniate (1620s) "maintain dogmatically or obstinately" (from Latin opinio "opinion"), a verb once used where now we use opine. Also see opinionated.
mid-15c., opinen, "express an opinion or opinions; to think, suppose," also transitive, "be of the opinion that," from Old French oppiner, opiner (15c.) and directly from Latin opinari "have an opinion, be of opinion, suppose, conjecture, think, judge," which is of unknown origin. It is traditionally considered to be related to optare "to desire, choose" (see option), but de Vaan's sources find the evidence of this weak. Related: Opined; opining.
c. 1600, "stiff in adhering to preconceived notions," past-participle adjective from opinionate. Now seemingly with more a sense of "obstinate in asserting one's opinions." Earlier words included opinioned (1580s) "attached to particular opinions;" opiniated (1590s); opinative (early 15c.) "founded only on personal opinion" (from Old French and Medieval Latin). Related: Opinonatedness.
<a href="https://www.etymonline.com/word/opiniated">Etymology of opiniated by etymonline</a>
Harper, D. (n.d.). Etymology of opiniated. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/opiniated