The kunai is a traditional Japanese handtool that was originally invented for farmers, by farmers, though their association with ninjas in folklore and modern media also gave them a reputation for being concealable and deadly weapons. Small Kunais can be thrown, while larger ones could be wielded as knives like in the game.
The Kunai is one of several poison-coated weapons in the game.
There is a cut throwable version of the Kunai ("wpn_prj_leaf") whose inventory icon still exists within the game's files. As there is already a poison-coated throwable weapon in the pack, it is unclear whether this version of the Kunai is a regular throwing knife or otherwise.
The throwable version's internal name is a reference to the anime series Naruto, where it is both a commonly-used weapon and a nod to the name of Konohagakure (木ノ葉隠れの里, Konohagakure no Sato, lit. "Village Hidden by Tree Leaves" or simply "Hidden Leaf Village"), the hometown of the titular protagonist.