Yo! Is this good Latin?

[Wander]

In reading the Marcus Didius Falco series by Lindsey Davis (set in Vespasian's Rome), she has the protagonist and his friend Petronius frequently greet each other with, "Io!"

I read that as "Yo!", and it certainly works in the writing. I'm just curious, because my Latin is weak at best and certainly not conversational.

Is "Io!" documented as a Latin or period greeting? Or is it just artistic license for something that probably existed, but was too informal to be recorded anywhere?

Inquiring minds want to know, and thanks.