A motorcyclist who prefers a beat up, well used, sometimes rusted, and usually matte black machine to a shiny new bike. The opposite of bling. RATs come in both the sportbike and cruiser variety. RAT sportbikes often bear heavily massaged engines and are quite fast despite their disheveled, held together by duct tape appearance.
Also Riders Association of Triumph. A factory sponsered Triumph owners group, similar to Harley's H.O.G.
That RAT ZX9 looks like it's about to fall apart, but its been bored out to a k and has a NOS bottle tacked to the subrame.