A cryptolect or cant is a language spoken and learned only by specific members of a group with the intention of using it to communicate unintelligibly from the perspective of those outside the group. The most well-known example is the Etruscan cryptolect, followed by Shelta/Gammon/Travellers' cant .
Polari (or Palare) is another example, made well-known in Britain by Julian and Sandy in the radio show Round the Horne.
