A real asshole. Somebody who either did something wrong; or is simply disliked in general.
Sean: Mike stole $50 from me yesterday
Reuben: Yeah; He's a real bowl smasher
Someone (usually called Eddie) who produces a really huge turd that is very loud and audible to the other occupants of the house.
"Wow, Eddie just smashed the shit our of our toilet bowl. He must hate our toilet as much as he hates people. He's clearly a no-good bowl smasher."