To start using Thieves Cant in game, type -th console command for Mith's Virtual Console.
Thieves Cant is a creation of thieving guilds. They developed a language of inconspicuous hand gestures, sounds and body positions to use for signalling secret messages on public. It became a valuable tool for stealing or abducting in groups, indicating commands, targets or dangers (such as guards), et cetera. It is a very basic and simplified "language" for quick signs and not meant for carrying on full conversations, but rather quick orders or warnings only. Thieves Cant should only be used when necessary, since rogues tend to guard this language carefully. They would not want others to "interpret" it and learn their secrets.
On Arelith this language is known by:
There are no phrasebooks in the game for the Thieves' Cant, therefore, only rogues may learn the language.