1) The couch you add to a dorm room, or other room with a lofted bed or bunk beds, so that when you come home drunk, you don't injure yourself climbing into bed.
2) Any extra couch or bed in a dorm room/apartment that doesn't have a designated owner, and is instead used by whichever drunk friend claims it first.
"I knew I wasn't going to be able to make it into my bed last night. Good thing I invested in this drunk couch."
"I woke up this morning with Kevin passed out on my drunk couch again."