A person who constantly rants about the dangers of alcohol, usually while you are drinking. This person will most likely start these rants, regardless of how much you are drinking. May also refer to someone who likes to literally take shits in beer.
Dean: You know how John died? He Drank too much! Do you want to die too? You know lots of people die every year from alcohol. Are you listening to me? Pay attention!
Andrew: Shut up Dean. Don't be a Beer Shitter.