Yemsa is a minor language of Ethiopia, the language of the former kingdom of the people who call themselves Yamma and were absorbed into Ethiopia in 1894; people, kingdom, and language are traditionally called Janjero or Zenjaro, an insulting Amharic term meaning ‘baboon.’ A book by G.W.B. Huntingford, The Galla of Ethiopia: The Kingdoms of Kafa and Janjero, contains the following description (quoted in Andrew Dalby’s Dictionary of Languages, p. 475) of a remarkable feature of the language:
The royal language of Janjero consisted of a special vocabulary for parts of the body, weapons, and verbs of action referring to the king. Thus “eye” in common Janjero is afa, but kema in the royal language; “eat” is ma in common speech, bos in the royal language; and “spear”, ebo in common speech, is me’a in the royal language. The language of respect used special words to describe the ordinary actions of notables: “eat” is ma in common speech, but ta in the language of respect. Improper use of the royal language was punished by death.
A special court vocabulary is an element of a number of East Asian languages (Japanese and Javanese spring to mind); I imagine there are other African examples, but this is the first I’ve come across, and in quite a remote little kingdom. Surely someone must have done a general study of this phenomenon.