Any person who consistently shows disregard for the normal social behavior of any given social situation, or has no concept of what the normal social behavior is for any given social situation. In addition, this person must evoke a feeling of empathy from within people that are more socially skilled than "That-guy." If this person does not evoke pity, tham he is just a jerk.
There is always that one guy at the dinner who is always there when you are, but you never get there before him and he never leaves before you do, so, you just kind of assume that this person lives at the dinner. And he'll know all the waitresses by name, and probably dropped out of college, but you don't really care enough to get the real story, and you already have enough neative vibes toward this guy because of his akwarness that it really doesn't bother you to write him off as a waste of life.