You're driving and get stuck in the middle of the intersection just as the light turns red. Now, because of you no one can get by. You have now become "That Guy".
The most annoying person at a social outing or gathering, especially when alcohol is involved.
Whatshisnuts- "Dude, So-and-so was being 'That Guy' at the party last night."
Theotherguy- "Yeah, especially when he keptscreaming then telling people to shut up."
An individual at any given social event who crosses the line of severe Obscenity/Indecency/Profanity typically involving speech or actions, often vulgar and unnecessary. Doing so defines his title of "That guy". See also; jackass, Andrew, Fatkid.