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This is a person that is pretty much a sarcastic asshole and/or a slightly balding fuck. They try to make you laugh by tellin terrible jokes about some dead guy that nobody gives a flyin fuck about in the first place. They cant stop talking about needless things in life.
Q: Hey i am trying to think of a description for Mr. Griffin. Can you help me?
A: Yeah man thats easy. He is That Guy.
Q: Isn't that kinda harsh?
A: Nah man he meets the description perfectly.

Example #2

Trinite's are as a whole "that guy"
by NotThatGuy2015 January 14, 2011
Someone who's always calling a girl, or IM-ing her, or running into her at a bar. He always flirts with her -- and even thinks he has a shot -- even though she's made it perfectly clear that she's not interested. Not as malicious as a stalker, just annoying.
Ugh, that guy called me AGAIN.
by Leefy Greans January 11, 2006
"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.
Morgan Freeman: "Look, someone is about to bowl another 300 game!"
Edward Scissorhands: "It's THAT GUY! Man, I hate him"
Morgan Freeman: "No you don't"
Edward Scissorhands: "You're right, I love THAT GUY"
by Moe0618 October 09, 2012
An Overall average looking guy, that dresses simply with a boring personality that is dating the hottest girl in the room.
Guy1: What does she drink? Pointing to a girl
Bartender: Be Careful she's sitting with her boyfriend
Guy1: ThatGuy? She's Dating Him? Really? Ok
by Jacob R. Daugherty December 24, 2008
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!
Nightshade is officially that guy
by Unknowni123 July 29, 2013
Term of brotherly advice, often given to oneself, to avoid acting like embarrassing dickwad as seen on YouTube or other social media.
“Did you see where Katherine Heigl’s husband got a tat with her zodiac sign and birthday?"

“Don’t be that guy.”

“Did you see where Rick Perry couldn’t count to three on the presidential debates?”

“Don’t be that guy.”
by Otisregrets November 24, 2011
In pretty much every country around the world where drinking alcohol is a cultural norm, every single social circle includes at least one person who becomes "that guy" (plural: "those guys").

We all know of whom I speak: The one friend who makes himself stand out in all the wrong ways. He always gets absolutely trashed, and makes a COMPLETE tit out of himself.
"Yeah, my friend Dom just got chucked out of the union for getting his knob out in the middle of the dance floor before unloading his stomach onto the floor"

"Shit. Doesn't sound good".

"Standard. He can generally always be counted upon to be That Guy".
by Zarathustratosphere June 26, 2011