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What's going on? What's happening? Often used as a greeting that does not require an answer. So what's up? How have you been? Well, what's up? Why are you so upset with me?
See also: up
Inf. Hello. What is happening? What's up? How're you doing? Hey, Chuck! What's up?
1 what’s the matter?: What’s up with him? He looks furious.
2 (especially American English) what’s new?; what’s happening?: Hey Jo, what’s up?