A REALLY ugly motorcycle that is ridden often, never cleaned, and maintained at the absolute minimum for it to run. Some rat bikes are built from scrap parts (often from 4 or 5 different motorcycle brands), others start off as nice bikes and gradually become rats. They are often spray-painted camo or matte black to cover all the rust.
I picked up a rat bike at a redneck garage sale for 50 bucks.
Urban sport bike ussually stripped down to bare essentials, painted flat black, and held together by zip ties and duct tape. Think Mad Max. Often the result of a nice new shinny bike being laid down by a squid and sold cheaply by the rider or insurance company. Common in the SF Bay area.
Did you see that guy on the rat bike in West Oakland doing a burn out in a puddle of burning gasoline?