The phrase "that guy" is generally used to describe the subjective "lowest common denominator" of humanity, with regards to a given situation, place, or event. For example, "you know, that guy who picks his nose during team meetings, there's always one", or "Oh my God, look at the him, in the Bermuda shorts and Hawaiian shirt, he's completely that guy!"

Yes, we've all heard the phrase before. It has probably existed, to some degree, since the dawn of time. However, everything changed, starting in 1994, with the movie PCU.

It was a movie on the borderline of amusing; it's generally nothing to write home about. However, it did spawn one famous quip of zen wisdom.
Droz: You're gonna wear the shirt of the band you're gonna go see. Don't be that guy.
The rest is history. The phrase stuck, and now it has become a fringe part of the vernacular.

...Kind of like the Truffle Shuffle.