see also: whazzup, whassup, whasabi!!, etc.
2. Wtf is wrong with you??
It's probably originated from the late 20th century, when people were astounded with the sky. The only thing that they would reply was "nothing"
Probably the first term to the decline of western civilization
1. Person 1: What's up?
Person 2: "Nothing" (most common response, maybe 99% of people say that)
2. Person 1: What's up?
Person: "The sky" (smart alleck answer: also: "the ceiling" or, "da roof")