late 14c., "a freedman, an emancipated slave," from Latin libertinus "condition of a freedman; member of a class of freedmen," from libertus "one's freedmen, emancipated person," from liber "free" (see liberal (adj.)).
Sense of "freethinker" is first recorded 1560s, from French libertin (1540s) originally the name given to certain pantheistic Protestant sects in France and the Low Countries. This sense partakes more of liberty and liberal than of the classical meaning (in Old French, libertin meant "Saracen slave converted to Christianity"). Meaning "dissolute or licentious person, man given to indulgence of lust" is first recorded 1590s; the darkening of meaning being perhaps due to misunderstanding of Latin libertinus in Acts vi:9. For "condition of being a libertine" 17c English tried libertinage; libertinism (from French libertinisme).