To urinate in a persons rectal cavity. Can be followed by "siphoning" in which a hoselike object is used to siphon out the urine.
After John gastanked Jill, he proceded to siphon out the urine with a straw.
You are stuck in a gastank when someone cuts a fart in an enclosed area, like a car or an elevator.
The drive to the beach sucked because Hugh had some serious beer farts and it was raining. I couldn't open my window and my eyes were watering in that gastank.