The item of marijuana paraphernalia that is evolved when one is so chronic that one brings ones gravity bong into the bath with oneself.
Usually in the form of a 2-liter soda bottle with the bottom cut off, the bathtub bong is always the piece in a marijuana smoker's collection that he has had the most fond memories with.
Because you can't call yourself a pothead if you haven't passed out in a bath full of warm bong water.
Stoner: "Yo, Brosephus Rex, where's my bathtub bong?"
Roommate of Said Stoner: "I was using it in the sink like a normal person, you fuckin' pothead."