This is to insinuate that somebody has sex with their exhaust silencer; normally on an ATV/Quad Bike. Although this is normally physically impossible due to the exhaust design it is a great put down to many engine enthusiasts.
Tom: You exhaust rapist , go shag your quad.
Dan: Go die you homo.
Although a quadbike is also known as an ATV it has an alterior definition when combined with the later syllables of Godzilla (the big monster). It can be used to offend somebody who is involved in the sport of Quadbiking. Quadzilla is combining the word 'quad' and 'zilla' to infer the person gains pleasure from quads.
Tom: Oi quadzilla, how tight is your exhaust?
Dan: Do one, you chunky bish. Hazel.
Tom: Ok calm down.