ETYMOLOGY
either from French opiniastre (adj. – opinionated) + -ous,
or from opiniastre (n. – an opinionated person) + -ous
EXAMPLE
“…Next, in matter of death, the Laws of England, wherof you have intruded to bee an opiniastrous Sub advocate, and are bound to defend them, conceave it not enjoyn’d in Scripture, when or for what cause they shall put to death, as in adultery, theft, and the like; your minor also is fals, for the Scripture plainly sets down for what measure of disagreement a man may divorce…”
From: Colasterion
A Reply to a nameless Answer against the Doctrine and Discipline of Divorce
By John Milton, 1645
Lexicophilia 