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
Party Goer "Yes I do indeed enjoy box wine."
That Guy "I go down on my fat ex girlfriend when its her time of the month"
Party Goer" Oh, so you're that guy, please get away from me"
That Guy " Waaaahhhh You think I'm a fat loser and I only like fat chicks."