Medieval Latin, from Late Latin Sclavus, from Byzantine Greek σκλάβος or Σκλάβος (Sklábos), probably from earlier Σλαβῆνος (Slabēnos), from plural Σλαβῆνοι (Slabēnoi), from Proto-Slavic *slověne (plural; the singular form Proto-Slavic *slověninŭ is derived from it).
The origin of σκλάβος has been disputed historically. Modern etymologists accept that it refers to Slavs (Old Slavonic словѣнинъ, словѣне), often enslaved during the early Middle Ages, and that the originally ethnic term came to have a more general social meaning, possibly around the 9th or 10th century when it appeared in German texts. An alternative hypothesis, now obsolete because it requires unexplained and unattested phonetic irregularities, is that it's from the Greek verb σκυλάω (skuláō), a variant of σκυλεύω (skuleuō, “to get the spoils of war”).
- ^ F. Kluge, Etymologisches Wörterbuch der deutschen Sprache. 2002, siehe «Sklave».