opiniaster (n.)

"one obstinate in asserting or adhering to his own opinions," c. 1600, from French opiniastre, from Latin opinio "opinion, conjecture" (see opinion) + deprecatory suffix (see -aster). Another word in a similar sense was opinator "opinionated person" (1620s), from Latin opinator "one who supposes or conjectures." Also opiniator (1520s).