Ninki is a general term for tools that were supposedly used by the Ninja (a secret agent in feudal Japan highly skilled in stealth and secrecy).
There are a wide range of Ninki that were considered to have been used. They are, for the most part, classified as the following. "Kaki" (firearms) that used gunpowder such as Noroshi (signal fire). "Suiki" such as Mizugumo that was related to water, of which their practicality is uncertain, but are thought to be used when a Ninja got over water. "登器," tools that were used to climb such as Kagibashigo, which was a hook attached to a rope and used when climbing buildings and mountains. "開器" such as Tojime that were used when escaping from an enemy, hitching to a sliding door to lock and gain time for an escape. "Buki" that were used to attack an enemy such as Shuriken.
Ninki was mentioned as old as in "Bansen/Mansen Shukai," a Ninjutsu (ninja techniques) strategy book which was written by Yasutake SAMUJI. Furthermore, books on Ninki are being sold at present as well.