Kunai was originally a tool which was treated as an utility knife or an all-purpose scoop in the general market at the time and thus ninja began to use it as a weapon because no one was suspicious of a person who carried a kunai with him.
元々は工具であり、当時の一般市場では万能ナイフか万能スコップ扱いで通るので、持ち歩いても不審に思われなかったことから忍者が武器として使うようになった。