a person who loves a particular thing, music group, show, etc. And views anyone who disagrees with him/her as not only wrong but their enemy. They view the disagreement as a personal attack and will usually resort to extremely defensive, and irrational form of arguing.
Steve: "Nintendo is for babies, Xbox kicks ass"
Allan: "You're a baby! Xbox is for pedophiles, Nintendo is awesome!"
Steve: "Go play with your Super Monkey Balls."
Allan: "You mean like your mom did last night?"
You can pretty much substitute the "Xbox" and "Nintendo" for anything from fruits to clothing products.