Kind of like you might say 'your humble servant' in English, the Venetians would say "sciavo vostro". Literally "your slave" - schiavo vostro in modern Italian. Which then morphed into "ciao".
At least in Alsace we sometime use "service" as a "you're welcome" equivalent instead of the more widespread "de rien", or "avec plaisir" you will ear in France.
Kind of like you might say 'your humble servant' in English, the Venetians would say "sciavo vostro". Literally "your slave" - schiavo vostro in modern Italian. Which then morphed into "ciao".