A “yo” sign? Well, if you mean shouting or texting “yo” in greeting, this has honestly come from rap music. A greeting used in rap music has now become an acceptable way to greet your friends. Some may not appreciate it, but there it is. However, there is a recording of the word “yo” being used in areas other than Spanish (where it means “I”) is in the middle ages in England.
Think about that for a moment. The word started in the middle ages as some kind of obscure word, and now it is a commonplace greeting among friends (or at least, a way to get attention from someone that you’re talking to). That’s practically all language is: a morphing of meanings of the words.
The yo sign, as far as I know, is an informal greeting or acknowledgement used by African Americans, adopted by rappers, and now used much more widely; or a more formal greeting in East Asia. It was possibly an old exclamation in England centuries ago.
