When something is so unfunny you then look on that person or thing with contempt or even anger. Also, something that is supposed to be funny but isn't so much that you become angry for having wasted your time on it.
Me: Dude, check out this video on YouTube!
(after watching video)
You: Wow, that was negative funny, thanks a lot dick! I wish I had my 5 minutes back!
The guy who always tries to make everyone laugh but is completely lame is negative funny.