'Sup meaning Wassup which then derives from "What's up?". In other words, "What is happening?".
This makes a great greating.
Nigger 2: shut yo mouth before i bust a cap in yo ass homie biatch!
Nigger 1: Nigger please
Nigger 3: 'sup my nigger brothers
Nigger 1/2: word
Many do in fact actually respond to the question with an answer but quickly disregard it and fray away from it... another way of responding to it would be (Ex 2.)
Ex 2.) "Sup jew?", "Hey man, havent seen your ass in a long time... wanna hang out on Saturday?"