Usage of a plastic bag, usually one left over from a previous grocery shopping event, to cover your bicycle's seat. This is done in order to protect and keep your seat dry, therefore, avoiding soaking the seat with water and mud.
Person 1: Dude, what's with the plastic bag on your seat? It looks funny.
Person 2: It's my bike condom. It keeps my seat from getting drenched and dirty on rainy days. This way my butt doesn't get all soggy either.