Ciao is one of the words we Italians use more often in a day, and is probably the most famous Italian word abroad (well, after pizza).  And yet, the origins of the word ‘ciao’ have nothing to do with familiarity as we use it today to greet someone we know well. Ciao derives from an ancient Venetian greeting, ’s'ciavo,’ that is ‘slave’ (implied: yours), which people used to express respect. 

’S'ciavo’ descends in turn from the Latin ‘sclavus,’ which has the same meaning and was used to indicate people of Slavic ethnicity, from which came the greatest number of slaves in the Mediterranean.