SERVI. This name was given by the Romans to their slaves; they were so 
called from servare, to preserve, from the ancient practice of the generals 
of the army, who were accustomed to sell their captives, and preserved them 
rather than kill them: servi autem ex eo appellati sunt, quod imperatores 
captivos vendere, ac per hoc servare, nec occidere solent. Inst. 1 3, 3.