A Bum Bike is any sort of bicycle that has been modified with a 2 stroke engine kit. These bikes are operated by a variety of people including the homeless population who has taken a liking to this quick and cheap form of transportation.
They are great for getting around town if you don't have a driver's license, the kits can be found for less than 200$. It's not uncommon to a see a bum or a junky ripping around on one with a few bags of soda cans and some stolen goods.
Hey Joe you better lock your tool shed I heard a bum bike zip by!
What's that sound !? Oh shit it's a bum bike! The junkies are out to steal catalytic converters again.