This is yet to be done, though a priority piece for both it’s popularity as well as its relevance to both my own life and research. For the time being I present the working defonition presented by Dr. Kacem Zoughari:
“In a great number of cases, the ideogram [shinobi] translates into the concept of patience, endurance and perseverance, sometimes even the idea of resignation, a kind of patience in the face of all tests. By breaking up the ideogram “nin,” one finds two other ideograms: “the heart, feelings, or spirit” (心), and the edge of the blade (刃). depicts the state of mind of one who suffers, endures misfortune, and must moreover hide, to hide one’s pain. In other words, to hone one’s thoughts and ego like the edge of the sword, to be constantly vigilant, forming one’s heart to endure. Such is one of the many meanings of the word shinobu (忍ぶ).”
-Dr. Kacem Zoughari
I have also written a related piece (also under construction), which can be viewed and read at my ResearchGate profile here.
Zoughari, Kacem. The Ninja: Ancient Shadow Warriors of Japan. (2010), pp 146