Origin of the Italian "ciao"
It's in Italian (by my favorite Italian podcaster) so I'll summarize: It comes from the word schiavo, which she says means "servant" but which I think also means "slave." (It looks like slave -- and resembles the French word for slave, esclave.) People would formally greet each other "Vi sono sc'iavo, meaning "I am your servant" (or, "to you, I am a servant"). This got contracted and contracted and eventually just became the very informal greeting ciao.