c. 1400, from Old French vexacion "abuse, harassment; insult, affront," or directly from Latin vexationem (nominative vexatio) "annoyance, harassing; distress, trouble," noun of action from past participle stem of vexare "to harass, trouble" (see vex).
word-forming element making adjectives from nouns, meaning "having, full of, having to do with, doing, inclined to," from Old French -ous, -eux, from Latin -osus (compare -ose (1)). In chemistry, "having a lower valence than forms expressed in -ic."
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Harper, D. (n.d.). Etymology of vexatious. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/vexatious