A keybanger is to coast along in an manual carby'd car with the engine off pumping the go pedal 3-4 times then with the car in gear dropping the clutch causing the car to 'roll start' causing a great bang to show you mates you need a new muffler.
OMG I shouldn't have done that keybanger, I need a new muffler now eh uleh?
A keybanger is a person that has limited success with their sexual escapades in real life, and therefore takes out their sexual frustration online, via trolling, tearing successful people down, or otherwise critical posts. They more vehement their postings, the more virginal they are likely to be.
Non-virgin1 - These kids on 4chan are pretty pathetic, eh?
Non-virgin2 - Yeah, these keybangers should get out of the house and try to bang a girl instead of banging their keyboards all day.
Non-virgin1 - Oh well, more hot tail for us!