1. An extremely pessimistic person who constantly complains about a problem but does nothing to fix it.
2. Somebody who spends most of their time dicking around on message boards or possibly on urbandictionary.com complaining about all the crappy things in their life.
3. A hilarious comic strip in MAD Magazine about this really pessimistic guy who makes everybody around him miserable.
Emo dude: I'm gonna go online and complain about how much my life sucks because my grades are crappy and I hate my parents and I have no friends!
Regular dude: Get off your ass and do something about it then, bitterman.