This is a functional knife made out of katsuobushi (dried, fermented and repeatedly smoked skipjack tuna). Katsuobushi holds the record for being the hardest food on earth, even though I've been eating rocks since I was a kid, no problem. Katsuobushi is typically shaved from a large piece with a special wooden box plane for use in soup stock or as a food garnish. Apparently it can also be repeatedly shaved and filed into a function knife. So, the next time you're stranded on a deserted island with nothing but a piece of katsuobushi, you can relax knowing that the shark you eventually decide to feed yourself to will get some extra nourishment from the piece of dried fish in your pocket, and that's a comforting thought.
Keep going for the video.
Thanks to Marcus O, who agrees anything can be a weapon if you sharpen it enough or it's large enough to stuff down an opponent's throat and cause choking.