English word "Slav" is actually very closely related with the word "Slava", which in old Slavic literally means "fame/glory".

Concerning slavery:
" slave - late 13c., "person who is the property of another," from O.Fr. esclave (13c.), from M.L. Sclavus "slave" (cf. It. schiavo, Fr. esclave, Sp. esclavo), originally "Slav" (see Slav), so called because of the many Slavs sold into slavery by conquering peoples.
This sense development arose in the consequence of the wars waged by Otto the Great and his successors against the Slavs, a great number of whom they took captive and sold into slavery. [Klein] - etymonline.com "