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1787, Latin, literally "of one's own kind, peculiar." First element from sui, genitive of suus "his, her, its, one's," from Old Latin sovos, from PIE root *swe-, pronoun of the third person (see idiom).
A person or thing that is unique, in a class by itself: “She is an original artist; each of her paintings is sui generis.” From Latin, meaning “of its own kind.”