The result of masturbation to the extent that one is found lying naked on their backs, flaccid penis in hand with partially soiled receptacle (sock, tissue, tp, navel, etc.).
Dude, don't walk into Matt's room, I just walked in and found him in a beat off coma and he looks like he saw a spooky ghost.
This term is used to describe that you're collegeroommate won't be in the room for a while which means you have plenty of time to rub one out. Usually these words are yelled out.
My roommate went home for the weekend so it's going to be BEAT OFF CITY in this piece!