The act of being in the trunk
of a car while it is in motion
. Occasionally it's required because of lack of proper seating, though most of the time it's a fun way to pass time amongst bored kids.
There are many fun things to do with trunk rides. The most obvious is of course, having the driver zig-zag, stop quickly, and take sharp turns. As long as the person in the trunk is ready for what is about to happen and their aren't any loose articles (like crowbars) in the trunk with him/her, this can be very enjoyable for the person in the trunk - like being on a roller coaster.