From the rec.sport.pro-wrestling FAQ (Oct 9, 1995):
15.7. What is Kay Fabe?
The term Kay Fabe comes from ancient carnival talk, appropriate as professional wrestling has it’s origins in the carnivals. Kay Fabe practices were old tricks, from three card monte to cure all elixers and, of course, magic acts. A kay fabe violator exposed the secrets behind these practices. In wrestling, the term has come to mean not exposing that the business is worked. In the 80s, Satoru Sayama, the original Tiger Mask, wrote a book entitled Kay Fabe, exposing many secrets of the business.