Someone who is so into the MMORPG he or she plays that they neglect the various aspects of their real life. When said person refuses to partake in a social activity proposed by a friends or loved one, the term can be used by said friends or loved one to hopefully modify the hoMMO's behaviour by causing them to feel shame.
Guy 1: I asked Ben to come out with us tonight, but he said he needs to stay in to get enough gold for some new armor or something.
Guy 2: What a hoMMO.
Guy 1: Affirmative.