Kunai (くない) is a common tool used by ninja. The word is transcribed into Kanji as 「苦内」 and「苦無」.
A double-edge tool shaped like a leaf. It was used for climbing walls and digging, and also used as a survival knife; it can also bludgeon an enemy. The rear can be shaped like a ring for string/rope. There is a common misconception that it is a throwing weapon, which merged from fictionalized content.