What is the answer to was sup?

The term 'was sup' is shorthand for 'what's up'; so the correct answer is whatever you are doing or whatever is going with you at the moment. You could be watching television, on the phone, cooking dinner, happy, sad, angry, or anything. Others like to give a more sarcastic answer saying things like 'the ceiling', 'the roof', and 'the sky', because those things are literally what is up.