A piece of trash moped that somehow runs even though it looks impossible.
A bike cobbled together out of many miscellaneous parts for next to nothing.
A bike that is poorly maintained, never cleaned, dropped many times, basically a beater.
A bike that is ridden forever, and replacement parts come from junkyards, auto shops, or home made parts. Usually end up getting everything painted one solid color like black, red, white, or pink. Usually the chrome gets painted over first.
Some people say rats are never made, while others make rats all the time. There are many different opinions of rats.
There are many different kinds of rats: junk rats, city rats, ratfighters, survival rats, basic rats, cafe rats, rat trikes, blacked out bikes, rat choppers, make a new style and add to the category :P
Example: Hippy's of the Hells Satans "Rat outa Hell"