c. 1200, rancour, "a nourished envy; bitterness, hatred, malice," from Old French rancor "bitterness, resentment; grief, affliction," from Late Latin rancorem (nominative rancor) "rancidness, a stinking smell" (Palladius); "grudge, bitterness" (Hieronymus and in Late Latin), from Latin rancere "to stink," a word of unknown etymology (compare rancid). Sometimes in 15c. medical works the word is used in English in its literal Latin sense.
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."
<a href="https://www.etymonline.com/word/rancorous">Etymology of rancorous by etymonline</a>
Harper, D. (n.d.). Etymology of rancorous. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/rancorous