Nushu (also called Dong language) is a fascinating secret language developed by women in a part of Hunan in harsh, old, patriarchal China. It was used to educate and evade repression, since women were largely forbidden from the realm of formal education. They would often write it in hidden spots in books, on fans, and even stitch it as embroidery in clothing. It is always amazing how far repressed people will go to not only survive, but educate and thrive.
Nushu is now, unfortunately, a dying language spoken by only a handful of women. It should be formally studied and preserved. I do want to write on this formally one day, but for now I am only including it in my fiction. Experts should study this before it vanishes as a living language.