One of the most cruel things you can do to a person, or rather, their toilet. The TankJob can only be pulled off on a toilet that has a tank on the back (many commercial grade toilets don't have one, thus are not an option). To complete it, you must remove the lid of the tank, and crap directly into it (the source of the clean water when the toilet is flushed). A particularly messy crap is best, however, if you have the balls to pull this off, ANY crap will do. Toilet paper NEVER makes it's way into the tank during the process... it ALWAYS is to be placed in the toilet bowl, where it belongs.
Ripped up on free beer at his cousin Steve's wedding, Glen decides that the country club it's being held at REALLY needs a TankJob. It's a hard choice between that and pooping on the practice putting green (which is lit and in full view of the dining room), but the club ultimately receives the TankJob.