This definition is actually referred to as "Fan boy"
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.
Allan: "Nintendo is better than Xbox"
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.
An idiotic fan of the anime Fullmetal Alchemist (which is actually really good), however, is so insanely obsessed with it that he will diss every other show and call anyone who doesn't like FMA an idiot.
igloo15 and bakudan are fullmetal assholes