The dumb guy down the hall who wears a fedora every day because he thinks it looks cool or something.
Fedora guy can be anyone with a really obvious and ridiculous affectation that makes him look more like the immature dork he is than the imaginary worldly and mature non-dork he sees himself as.
"In-the-know" college girl: "So I was like, 'Epic fail!'"
Fedora Guy: "Did I just hear 'epic fail'?" -winks at girl-
-room goes silent-
Person 1: "Fedora Guy is such a dork."
Person 2: "I walked in on him playing Dungeons and Dragons with his fat girlfriend the other day."
Person 3: "Seriously. But, hey guys, how do you like my new American Apparel Deep V tee?"
Persons 1 & 2: "Quit being such a fedora guy before we fart on your head."