‘Nushu’ was a writing system created and used exclusively by women in the Jiangyong county of Hunan province in China. Culturally prohibited from any kind of formal education, girls developed Nushu over hundreds of years.
The Nushu script is more joined-up than written Chinese. Unlike the standard written Chinese, where each written symbol or 'grapheme' represents a word, Nushu is phonetic, with each of its approximately 600-700 characters representing a syllable in the local Chéngguãn dialect
Jiangyong county has a culture of 'sworn sisters' who pledge friendship to one another forever. When a girl was entered into a forced marriage, she would be given a diary by her 'sisters' containing messages of good luck written in Nushu. The rest of the diary would be empty and she would use it to record thoughts that she wished to keep secret from men.
Many of the Nushu diaries were embroidered rather than written; the women were unable to get far away from home due to the difficulty in walking with bound feet, and so had plenty of time to write in the diaries.
Many women in the far east have 'slush funds' which they keep secret from their husbands (38% of Japanese women, slightly less in China, and up to 65% of married Korean women).
For 87 years, the Portuguese kept the existence of Ascension Island a secret from the rest of the world.
A secret language, or 'ludling', is a system of manipulating spoken words to render them incomprehensible to the untrained ear. The following language games involves a usually simple standard transformation to speech to encode it.
In Pig Latin the initial consonant sound of an English word is transposed to the end of the word and the sound 'ay' is added. The Pig Latin for 'Quite Interesting' is 'Itequay Interestingyay'.
In Gibberish, one adds an infix such as therg-, -elag-, -itug-, -uthaga- -idig- or -atheg to each syllable. Around the world there are plenty of other secret languages, including the Afrikaans ‘P-tall’, the Bulgarian ‘Pileshki’, the French ‘Louchebem’, the German ‘Löffelsprache’ and the Japanese ‘Ba-bi-bu-be-bo’.
The USA's National Archives contain recipes for invisible ink from the time of the First World War. They are still secret.
Ninety-two percent of the stuff told to you in confidence you couldn't get anyone else to listen to.