When the steering wheel was first put into use in automobiles, it was called the "whip". The whip is what you used to control the horses on a stagecoach, hence the analogy. Many years later, various hip hop artists noticed that the Mercedes-Benz logo resembled a steering wheel. They then proceeded to use the old term "whip" to describe any Mercedes-Benz vehicle. The term has now been generalized to classify any expensive automobile.
A whip has simply come to represent a car. It has been said that it was used a long time ago when a whip would be used to steer the horse-drawn carriages. It is used in hip-hop fashion, and it has come represent any car not neccesarily a really nice car.
"Friend #2: Yo, Eric, I got a new whip.
Eric: Aww, foreal? What kind, man?
Friend #2: A Civic. But it doesn't work right now.
Eric: What's wrong with it? ..."
"Now I'm super fly slick without a roof on my whip..."
-Lloyd Banks