late 14c., "a freedman, an emancipated slave," from Latin libertinus "member of a class of freedmen," from libertus "one's freedmen," from liber "free" (see liberal). Sense of "freethinker" is first recorded 1560s, from French libertin (1540s) originally the name given to certain Protestant sects in France and the Low Countries. Meaning "dissolute or licentious person" first recorded 1590s; the darkening of meaning being perhaps due to misunderstanding of Latin libertinus in Acts vi:9. As an adjective by 1570s.