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1.
An attention seeking idiot who actually thinks it's cool to be the first person to comment on an Internet article, video, post, blog, etc.

Sometimes, multiple fags do it at the same time, only to look like idiots, as shown below in ex.1.
ex. 1
news story's comments:
-FIRST!
-FIRST!
-FIRST!
-stop that shit it's getting annoying.

ex.2
some celebrity's facebook page:
-first!!!
-the guy who posted 'first' is a First Comment Fag! thumbs up if you agree!
by glugugugugugug February 20, 2011