This article is from the Artificial languages FAQ, by Rick Harrison firstname.lastname@example.org with numerous contributions by others.
Individuals and groups will sometimes invent secret languages to
keep information private from persons who have not been
initiated. Pig Latin, used by some English-speaking children, is
probably the most famous example, but it is little more than a
re-arrangement of the phonemes in English words; other cultures
have produced secret languages that are more effective at
concealing information. Damin is an example.