When you take a massive shit. Especially one that fills a large volume of the toilet bowl and leaves the shitter with a strong sense of accomplishment.
Person #1: Dude, how was your shit?
The Shitter: Ah, it was quite bowlfilling.
Boys who enjoy yaoi (a genre in Japan that contains sexual and/or romantic relations between two men); literally translates to "rotten boy"; corresponding female : fujoshi