A person who pretends to hide his/her drinking problem, yet everyone knows what a drunk mess they are.
Brandi likes to take shots alone in her room, then come out and stumble around the apartment. She's such a closet drinker!
Person who sits in his/her home and consumes alcoholic beverages in a manner that would otherwise be considered a party. Also look up "loser" and "virgin".
That guy is such a closet drinker.