Joss is the young boy from Aesop's fable "The Boy Who Cried Wolf" and can be encountered in the Land of a Thousand Fables. True to his tale, he lies all the time.
If Geralt enters the illusion: Geralt, along with Syanna, encountered Joss who was naturally lying as always. After Syanna explained this to the witcher, the two then used the information Joss knew about the magical beans to figure out their real locations.