A kunai is a Japanese multi-tool, often depicted as a handheld or thrown dagger. Traditionally, only the tip is sharpened, while the sides are dull; in most fiction and modern reproductions, however, the blade is sharp all the way around. Prior to its usage as a weapon, it was mostly seen as the Japanese equivalent to a trowel when used in gardening or masonry.
The kunai is very much ingrained in tales of ninja and its ubiquity is synonymous with the shuriken. As a prominent work of fiction involving ninja, Teenage Mutant Ninja Turtles have a number of characters that utilize the weapon.