"ingenuous, artless, natural," 1590s, from French naïf, literally "naive" (see naive). The masculine form of the French word, but used in English without reference to gender. As a noun, "natural, artless, naive person," first attested 1893, from French, where Old French naif also meant "native inhabitant; simpleton, natural fool."
The neuter genitive pronoun in Middle English was his, but the clash between grammatical gender and sexual gender, or else the application of the word to both human and non-human subjects, evidently made users uncomfortable. Restriction of his to the masculine and avoidance of it as a neuter pronoun is evidenced in Middle English, and of it and thereof (as in KJV) were used for the neuter possessive. In literary use, his as a neuter pronoun continued into the 17c. In Middle English, simple it sometimes was used as a neuter possessive pronoun (c. 1300).
late 15c., "immoral woman," later (16c.) also "rascal, knave" (regardless of gender); probably from a continental Germanic source, compare Middle Flemish sloovin "a scold," sloef "untidy, shabby," Dutch slof "careless, negligent," Middle Low German sloven "put on clothes carelessly," from Proto-Germanic *slaubjan, from PIE root *sleubh- "to slide, slip." Meaning "person careless of dress or negligent of cleanliness" is from 1520s. Also see slut.