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]
Old English Wealh "Briton" also began to be used in the sense of "serf, slave" c.850; and Sanskrit dasa-, which can mean "slave," is apparently connected to dasyu- "pre-Aryan inhabitant of India."