Is it ensure, insure, or assure?
late 14c., Sclave, from Medieval Latin Sclavus (c.800), from Byzantine Greek Sklabos (c.580), from Old Church Slavonic Sloveninu "a Slav," probably related to slovo "word, speech," which suggests the name originally identified a member of a speech community (cf. Old Church Slavonic Nemici "Germans," related to nemu "dumb;" and cf. Old English þeode, which meant both "race" and "language").
Identical with the -slav in personal names (e.g. Russian Miroslav, literally "peaceful fame;" Mstislav "vengeful fame;" Jaroslav "famed for fury;" Czech Bohuslav "God's glory;" and cf. Wenceslas). Spelled Slave c.1788-1866, influenced by French and German Slave. As an adjective from 1876.