The word slave comes from the Byzantine Greek σκλάβος, via Middle Latin Sclavus, from which root Italian gets schiavo, French esclave, and Spanish esclavo. The original meaning of the word was ‘Slav’, as in ‘Slavic’: apparently a certain Otto the Great was such a prolific vanquisher and enslaver of Slavic peoples that the two terms became synonymous. Σκλάβος approximates the Slavs’ own name for themselves, the Slověnci, which survives in the English Slovene and Slovenian.
(The appealingly creative theory that connected ‘slave’ to the Greek verb skyleúo ‘to strip a slain enemy’ has been disproved.)
Interestingly, an analogous etymological tendency can be observed in Old English: around 850, the word Wealh meaning Briton (the original inhabitants of the British Isles, subsequently colonised by the Anglo-Saxons) began to be used in the sense of ‘serf, slave’.
The Slavic word for slave (Russian rab; Serbo-Croat rob) provides the root of the word ‘robot’, and shares an etymological lineage with ‘orphan’. Robot itself was coined by Czech writer Karel Čapek in his 1920 science fiction play R.U.R. (Rossum’s Universal Robots).