Wassup means whats up. Whats up is a way of asking how some one is, and or what someone is doing. Se Wassssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssup.
Most people with an IM start a conversation by saying Wassup, or its shortining sup. Sup can mean the same thing, or a random thing. Only in America