late 14c., Sclave, from Medieval Latin Sclavus (c. 800), from Byzantine Greek Sklabos (c. 580), from Proto-Slavic *sloveninu "a Slav," probably related to *slovo "word, speech," which suggests the name originally identified a member of a speech community (compare Old Church Slavonic Nemici "Germans," related to nemu "dumb;" Greek heterophonos "foreign," literally "of different voice;" and Old English þeode, which meant both "race" and "language").
Max Vasmer, the authority for Slavic etymologies, rejects a connection to *slava "glory, fame," which, however, influenced Slav via folk etymology. This is the -slav in personal names (such as Russian Miroslav, literally "peaceful fame;" Mstislav "vengeful fame;" Jaroslav "famed for fury;" Czech Bohuslav "God's glory;" Latinized Wenceslas "having greater glory"), perhaps from PIE root *kleu- "to hear."
In English, it was spelled Slave c. 1788-1866, influenced by French and German Slave. As an adjective from 1876.