When you are observing an attractive person (of either sex), and usually, but not necessarily their ass, and you become distracted enough that you run into something, either in your car or while walking.
1. The correct spelling of ass dent, an assodent is when--accidentally or not--several events lead up to a dent the shape of one's ass in the trunk of a car.
2. A dent shaped like an ass on a car's trunk.