Calling someone "That Guy" is saying that they would obviously be the one that would do a particular thing. This reference can only be made after the event has happened.

The guy who goes to a high school party after he's graduated high school.

He fools you once, you're mad. Fools you twice, how could he. Fools you three times and he's officially that guy, you know the one. He goes to the bar and he's like "This suit is officially Georgio Armani, my dad knows him", FUCK YOU. I AAAAAIIIIINN'T HAVIN THAT SHIT!

1. (n.) The guy who wears the t-shirt of their own band. 2. (n.) The guy who wears the shirt of the band that he has come to the show to see. 3. (n.) The guy who buys a shirt at a show and puts it on over whatever shirt they were previously wearing.

thers a "that guy" in every situation, he could be that guy who ate the last slice of pizza, or that guy who hit on your girl friend, that guy who who begs for money at lunch, or that guy who FUCKING SHOOTS DOWN YOUR GOD DAMN CHOOPER GUNNER WITH 3 KILLS TO A MOTHER F-ING NUKE!

a person who gives himself nicknames, laughs at his own jokes, and goes around acting like he's cool.

Someone who shows up at a party with a guitar in a pitiful effort to attract the opposite sex.