Kunai (くない or 苦無 in Kanji) is a multipurpose tool from ancient Japan. It originated during the feudal era and was primarily used by carpenters and other craftsmen.
The Kunai has a long, wedge-shaped, double-edged blade. It is forged from softer metal and is not very sharp because it was frequently used to cut and work with hard materials such as wood, bricks, and even bones. If it were made from harder steel, there would be a risk of breakage since hard steels tend to be more brittle than softer ones.
The impact forces generated during hacking and throwing would cause chipping along the edge or the tip to break if the blade were brittle. The handle is wrapped with bands, and at the end of the handle, there is a ring to which a rope with a weight or iron ring could be attached, creating a Kunai-fundo.
Since the Kunai was a commonly used tool, it was easy for ninjas to carry it discreetly and utilize it as a weapon. They would either carry it in a leather sheath attached to their obi (belt) or diagonally bound across the body with a rope.
Due to these circumstances, the Kunai proved to be an effective ninja weapon and became popular among them. Ninjas could use it as a burglary tool, a climbing aid, or a weapon.
It was employed similarly to a Tantō but differed in being double-edged, allowing for additional applications. Additionally, it could be used for digging, stabbing, cutting, striking with the handle end, and prying. Sometimes, it was used as a throwing knife and spearhead.