A hobo beer is an old beer that someone did not finish. Once found the next day, especially in scary apartment basements, they are often left out in the middle of the room, as hobo bait. The logic being, if a hobo came down there, he'd drink the beer for sure. Then we'd have to whap him upside the head and straitjacket him. On acid.
Only dumbasses drink hobo beers, accidently or otherwise.