Used when you don't know the person's real name, but can be used even if you do, just to replace their name.
Donnie: "Hey, Mariano! I'm gonna nab me a brat. Just gotta stop by da cash machine first. You want one?"
Mariano: "Sure thing, joe! With extra mustard."
“Joe” meaning “generic man or fellow” dates back to 1846. It was derived from the biblical Joseph, the son of Jacob and Rachel. Nowadays, it is used most frequently by African-Americans in Chicago.
In Visual's “What's up Joe,” from the album Working Class Legend—Overtime (2010), they sing “What's up Joe. That's what they says.”