Someone who constantly looks for an excuse to use their knife to accomplish trivial tasks that could easily be solved using you bare hands/teeth.
Dave: "Oh sweet a pack of skittles!"
*Pulls out knife from belt clip*
Joe: "Dude, you realize that doesn't require a knife, right?"
Dave: "Yea but then I won't look like a badass."
Joe: " You, my friend, are a knife fag."