A person, usually male, that has a twisted sense of what is acceptable in society. They frequently engage in acts or say things that are completely outside the norm without even realizing that they are doing so. Typically, it is clear to this person's friends that he is a sick guy however the sick guy himself may not know it. Similar to a crazy a-hole however a crazy a-hole engages a sick act here and there while the acts of a sick guy go down to the core of his being.
"Why did you eat that piece of bacon that had been sitting out for three days. You're such a sick guy."
The guy who drinks the leftover beers left out overnight the day after a party. He's a sick guy.