Copied from a Blog called Sexy Spanish Club in Buenos Aires
http://sexyspanishclub.blogspot.com/2006/12/chau-means-i-am-your-slave.html
"Anyway, one interesting thing I read in 'Eat. Pray. Love.' by Elizabeth Gilbert was about the origins of the standard goodbye we say/hear here in Argentina: Chau. It is, like the Portuguese Tchau, based on the Italian Ciao and you might be entertained to know that it is an abbreviation of a medieval Venetian intimate salutation: "Sono il suo schiavo!" which means "I am your slave!"

Thought you might want to know.
Chau