c. 1300, "noxious animals," from Anglo-French and Old French vermin "moth, worm, mite," in plural "troublesome creatures" (12c.), from Vulgar Latin *verminum "vermin," possibly including bothersome insects, collective noun formed from Latin vermis "worm" (from PIE root *wer- (2) "to turn, bend"). Extended to "low, obnoxious people" by 1560s.
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 verminous. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/verminous