Oral Cipher in Yoshiwara Pleasure Quarters in Edo

Women in the Yoshiwara pleasure quarters in Edo in the late eighteenth century used a cipher, called the Fukagawa Speak, in their conversation. I learned this from a contemporary novel depicted in a scene in a historical drama I watched on TV the other day. The scheme is simple enough: meaningless syllables "ka", "ke", "ki", "ko", "ku" are inserted in the message. But when used in conversation, the resulting sound would have been unintelligible to untrained ears.
Similar schemes, typically known as the tanuki cipher, have also been used in riddles for kids. (Tanuki means a raccoon dog, but may also be interpreted as "drop ta".)

And, of course, it's not limited to Japan. Even John Wilkins' Mercury (1641) mentions an example of a similar scheme: Ougour plogot igis digiscogovegereged.
A Japanese translation of a mystery novel in English uses the Fukagawa Speak to render secret conversation in "King Tut English" among kids: A-bub-shush-o-lul-u-tut-e-lul-ylum.

I described these in a new article in Japanese, 深川言葉　―　大河ドラマ「べらぼう」に出てきた遊里の暗号.