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 15, 2011
Refers to Call of Duty: Modern Warfare 2. In every game you build up a killstreak, that eventually gets shot down by some jackweed. This jackweed is known as "That Guy", or someone who spends the whole game shooting peoples hopes, dreams, and AC-130's down.
That Guy shot down my chopper gunner before it could even get in the map. Now I have to get 14 kills by myself to get the nuke.
by Camp of Duty July 8, 2010
(n)A person who makes a fun occasion awkward or otherwise boring by being the one guy to disagree with the majority; a person made into an outsider by his ridiculous antics.
(v) being That Guy on any occasion; Being made into an outsider by your ridiculous antics.
"dude look at Rick"
Rick-(proceeds to tell the girl he just met about the history of Judaism)
"Oh god he's become that guy"
"Should we leave him?"
"Yeah, he's already too far gone."

"Bob, you've been That Guying on several occasions for a while now, we all just want the old Bob back!"
Bob-"you guys just don't understand! I REALLY LIKE NINE INCH NAILS!"
Rest-"oh god he just That Guyed so hard it gave me ASL"
by JackJackAttack September 21, 2014
a phrase used to describe a person at a party who tries to start drama, fight people, say gay things to other guys, or just makes an ass of himself.
bro1: dude johnny is downstairs trying to fight everyone and he told me that i was the sexiest man alive......

bro2: he's "That Guy" tonight
by bromontana179010 February 20, 2010
a term given to a person who is in fact there, yet not there in person. the kind of person who when you see you're all like: Wow, he's in this class? and when they leave, you don't notice their absence.
person 1:WHO THE F*** IS BOB?
person 2:He's that guy in our geo class dude...
person 1:OH!!! that guy!
by JeremyD December 7, 2006
(n.) The guy who walks around only killing his own team mates while playing video games.
That guy who kills you everytime you cross his path during Halo
by Hencakes December 17, 2008
Co-Anchor to the Facebook show.. Bout that Life TV.. thee name is Kevin Collins you already know dude knows what lifes about... let that thang wobble yerd
Your "that guy" when you go back after that girl
by Yerd September 17, 2010