A car tailor made for a creepy person, or "creeper"; typically a large Chevy or GMC van, sometimes a converted heating/air conditioning van, typically white, with most or all windows either tinted or covered with those bizarre opaque panels. Ideal for kidnapping young victims and transporting them across state lines. Taken from the name of infamous pedophile boaster John Mark Karr who most likely, if he were allowed to drive, would drive a john mark car.
Dude, look at that john mark car!
Some creeper with a pedosmile! Let's bounce before we end up in that dude's basement!