Someone who is talked up by everyone as a person who is awesome, but actually sucks in reality. They are not actually a man of honor, they are actually a piece of shit. When someone says that someone else is a great man, they don't actually mean that they are a great person, it is meant as a sarcastic term.
Also, when someone is constantly a "great man", you can award them the "great man award". This is given to them for simply being a great man!
John: (talking about people they know from the same high school) Do you know John Smith?
Mark: Oh yeah, John Smith, great man!
Bill: Did you hear about that asshole Phil?
Craig: Yeah dude, great man