1787, from L., lit. "of one's own kind, peculiar." First element from sui, gen. of suus "his, her, its, one's," from Old L. sovos, from PIE base *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.”