A bowl filler is the name assigned to an enormous amount of feces deposited into the toiled during a single act of defecation.
Phil: So was it a bowl filler?
Tom: You bet. The janitors needed to use the plunger to get it all down.
When you blast out such a quantity of shit that you literally fill the toilet bowl. Depending on the size of the particular bowl, you may or may not be sitting in your own pile by the end.
Honey, I knew I shouldn't have eaten those three buffets yesterday. I just had a bowl filler and clogged up the bedroom toilet. Now I need to call a plumber to get all that shit down.