The “GeekOut” t-shirt is a derivative of the Tap-Out brand (not affiliated). To “GeekOut” means to be extremely elated, but can only occur to the geek inclined. Like the feeling you get when, while flipping channels, you find that an episode from the original Star Trek series is on! Or the feeling you get when you realize that it's one of your favorite episodes (the GeekOut increases by at least 40%)!! Or the feeling that you get when you view your favorite scene from your favorite Star Trek episode!!!!!!!!!!!!!!.
Of course, you can GeekOut from other experiences, as long as they are geek.