The term is thought to derive from the names of characters that resemble the three strokes in the kanji character for woman (女 onna?); said in the order they are written: ku (く) - no (ノ) - ichi (一). Early literary quotes include Enshū Senkuzuke Narabi Nihyaku In (遠舟千句附并百韵?) (1680) as well as Maekuzukeshū (前句付集?) (1716), which specifically associates the word with the kanji 女 supporting the etymology. The "くノ一" writing requires the use of one character from each Japanese system of writing — first hiragana, then katakana, then kanji. While hiragana and kanji can exist in the same word, katakana generally cannot appear in conjunction with the others. There are exceptions to this, e.g. "ゴミ箱", "消しゴム".
A popular etymology would derive the term from 九能一 (能 "nō" : talent) with Japanese numbers "ku" (九) for "nine", the particle "no" (の) for "and" and "ichi" (一) for "one", literally translated to "Nine and One". The meaning for this name is derived from the number of orifices on a female body. A male has nine, a female has one more (vaginal opening) and possesses the skills to make use of this orifice as well. Another theory asserts that the term is apocryphal and coined in the writings of Ninpōchō novelist Futaro Yamada.