Is it ensure, insure, or assure?
late 15c., "servant, serving-man, slave," from Old French serf "vassal, servant, slave" (12c.), from Latin servum (nominative servus) "slave" (see serve). Fallen from use in original sense by 18c. Meaning "lowest class of cultivators of the soil in continental European countries" is from 1610s. Use by modern writers with reference to medieval Europeans first recorded 1761 (contemporary Anglo-Latin records used nativus, villanus, or servus).