The novelist Shinjuro TOBE said in his book, 'Ninja To Ninjutsu' (Ninja and the Art of Ninja), that kunoichi originally came from 'kuichi no michi,' which indicates the art of the chamber bed in Onmyodo (way of Yin and Yang, an occult deviation system based on the Taoist theory of the five elements), and the Chinese character '九' and hiragana, whereby 'く' happen to have the same pronunciation and with the combination of '一' it becomes '女.'
作家の戸部新十郎は、元々は陰陽道における房術を示す「九一ノ道」が本義であり、九の字がたまたま、平仮名のくの字と同じ発音のうえ、一と合すれば、ともに女になることからくノ一というふうになったものである、と著書「忍者と忍術」で述べている。