What’s the origin of “yo”? – English Language & Usage Stack Exchange

In the Neapolitan dialect “guaglione” (pronounced guahl-YO-nay) signified a young man. The chiefly unlettered immigrants shortened that to guahl-YO, which they pronounced whal-YO. That was inevitably further shortened to yo. The common greeting among young Italian-American males was “Hey, whal-YO!”, and then simply, “Yo!” And so it remains today.

https://english.stackexchange.com/questions/40408/whats-the-origin-of-yo