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1.
The obnoxious practice of responding to a blog post/article/video with a comment saying "first." Generally this comment will have no substance; it is about nothing other than itself. When the first spot is stolen by a non-firster a firster will often resort to "second!" or "third!"
Blogger17: Long, thoughtful post about something important

obnoxiousname666: first!
annoyedbyidiots42: damn firsters, firsting all over the place
by fallentomato July 06, 2009
 
2.
Fucking your first cousin.
There weren't any bitches left at the party, so I decided firsting was my only option.
by fav January 18, 2007