Medieval Latin, from Late Latin Sclavus, from Byzantine Greek Σκλάβος (Sklábos), probably from the Greek verb σκυλάω (skuláō), a variant of σκυλεύω (skuleúō, “to get the spoils of war”). [F. Kluge, Etymologisches Wörterbuch der deutschen Sprache. 2002, siehe «Sklave».]
The origin of σκλάβος has been disputed historically. Modern etymologists accept that it refers to the spoils of war, as it makes sense morphologically, but there is an obsolete theory that the word comes from plural Σλαβῆνοι (Slabênoi), from Proto-Slavic *slověne (plural; the singular form Proto-Slavic *slověninъ is derived from it), although this theory requires unexplained and unattested phonetic irregularities. It is argued that that the originally the term referred to Slavs (Old Slavonic словѣнинъ, словѣне), who were 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 - but nowadays it is commonly regarded that it was only old German propaganda pseudo-etymology.