A tool is one of those stupid assholes at parties who does nothing but try to bang everything he sees because he thinks he's wonderful. This is typically caused by The small amount of muscle definition he has managed to obtain during his time away from being an alcoholic. He is cocky, but deep down, he knows he is pathetic.
A tool will usually say things like "hey sweetheart, check out my cross tattoo," or "I wish I could've been here sooner, but I was working out."
The guy who smashes beer cans on his head is such a fucking tool.
The guy who awkwardly screams "HEYYYY" when someone enters a room is a tool.
The guy with a cross tattoo but holds no actual biblical knowledge is a tool.