This phrase, commonly attributed to the diminution of "Wassup?", the abbreviation of "What's up?", is really a Restaurant Phrase, which has now become part of our culture. It was used by waiters to welcome customers, offering Soup as en entry. As most of the waiters were immigrants, they had pronounciation problems and pronounced it 'Sup.
Evolution of "'Sup?"
1940: -'Sup? -Learn to speak, you filthy immigrant!!!
1960: -'Sup? _No, thank you.
2000: -'Sup? _Hey, yo!