late 14c., "belonging to a beast," c. 1400, "having the qualities of a beast," from Old French bestial (13c.) "relating to animals; beast-like, stupid, foolish, brutal" and directly from Latin bestialis "like a beast," from bestia (see beast). Sense of "below the dignity of a human" in English is from c. 1400, and in many cases does injustice to the beasts. When the beast of the Book of Revelation was meant, the adjectival form bestian (1650s) sometimes was used.
word-forming element making abstract nouns from adjectives and meaning "condition or quality of being ______," from Middle English -ite, from Old French -ete (Modern French -ité) and directly from Latin -itatem (nominative -itas), suffix denoting state or condition, composed of -i- (from the stem or else a connective) + the common abstract suffix -tas (see -ty (2)).
Roughly, the word in -ity usually means the quality of being what the adjective describes, or concretely an instance of the quality, or collectively all the instances; & the word in -ism means the disposition, or collectively all those who feel it. [Fowler]
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Harper, D. (n.d.). Etymology of bestiality. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/bestiality