c. 1300, sclave, esclave, "person who is the chattel or property of another," from Old French esclave (13c.) and directly from Medieval Latin Sclavus "slave" (source also of Italian schiavo, French esclave, Spanish esclavo), originally "Slav" (see Slav); so used in this secondary sense because of the many Slavs sold into slavery by conquering peoples.
https://www.etymonline.com/word/slave
slave(n.)
c. 1300, sclave, esclave, "person who is the chattel or property of another," from Old French esclave (13c.) and directly from Medieval Latin Sclavus "slave" (source also of Italian schiavo, French esclave, Spanish esclavo), originally "Slav" (see Slav); so used in this secondary sense because of the many Slavs sold into slavery by conquering peoples.