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 Narabini Hyakuin (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 character set — 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, for example in "ゴミ箱" and "消しゴム".
Female ninja are mentioned in Bansenshukai, a 17th-century Japanese book compiling the knowledge of the clans in the Iga and Kōga regions devoted to the training of ninja. According to this document, the primary function of female ninja was espionage, finding legitimate service positions in the households of enemies, to accumulate knowledge by gaining trust or overhearing conversations. One historically accepted example of this is Mochizuki Chiyome, the 16th century noblewoman with ninja roots who was tasked by the warlord Takeda Shingen with recruiting women to create a secret network of a few hundred female spies.