One who dedicates one's self to a particular sport to such an extreme it becomes annoying.
John: Yeah, I went to baseball practice yesterday. Did you?
Alex: Of course! It's only the best sport in the world! In fact, I just ordered three jerseys off of a baseball equipment website, and I subscribed to three wooden bat magazines last Monday. I'm still looking for an outfielder's glove and a catcher's mitt, though.
John: Dude, chill. You such a blatch about baseball.