The one person who, with a broken-up couple in a crowded room, says something loudly that would imply that the couple is dating.
This inevitably happens for each badly ended relationship once word spreads to everyone except "that guy" that they are through.
"That guy" refers to the narcissist inside each and every man. "He" is "That guy" when he gives into the urges (usually pertaining to sexual gratification) to the detriment of those around him. Nobody wants to be "That guy".
"That Guy" is the guy that you know of, but do not know personally. You see him at the bowling alley every week and he is just the sliest old man you've ever met. That guy has bowled 300's on several occasion which makes you want to know him even more. He's an asshole and as much as you want to hate him you cant, because he's THAT GUY. Just when you think you've had enough of THAT GUY, he openly makes fun of a person you dislike and he draws you in even further.