"Sup" is slang, a contraction of "what's up?," though it usually simply substitutes for a greeting formality such as "hi," "hey," or "yo," as 'what's up' has been wont to do. It is most commonly used by youth in very informal contexts. It does not require an actual response and is usually used for efficiency's sake instead of to convey semantic information; if speakers actually wish to inquire about another, they follow up the "sup" with "what's going on," "how's it hanging;" "how're you doing?" Related sayings are wazzzup and wassup, which are more expressive but take longer to say.