Ass, posterior, concentrating mostly on the anus/rectum. Where
the feces (poop) comes
out. The Storage area for Feces. Thus a "storage box' for feces.
"After all that tequiila
and Hot Sauce I will need to install
a new "shit Box"
"So it slipped out of her vagina and before
I new it i was up to my balls in her "shit Box".