1. To drive around in a nice car with the top down.
2. Generally living large; living life well.
3. That sexy activity endorsed by dental hygienists and other anal-retentive folk.
comes from the imagery of showing off your grill. you bare your teeth as if you were in front of a mirror flossing. thus showing off your grill came to be known as flossing and flossing came to mean showing off in general