This Gerry Anderson FAQ site: http://members.aol.com/kimmurphy/private/gerryfaq.html
11. What does "F.A.B." stand for?
Perhaps the most frequently asked question by new THUNDERBIRDS fans, the definitive answer came from Sylvia Anderson herself during her promotional tour for her book YES, M'LADY. According to Sylvia, F.A.B. stands for...
It was something they made up on the spur of the moment during a writing session, Sylvia says, and it was never intended to stand for anything other than to spell out the letters of a popular British slang word--"fab"--during that era. Many people have attempted to find other meanings ("Filed, Actioned, Briefed", "Fine--Acknowledge Broadcast"), but it never meant anything other than "fabulous".
It was just a cool British slang word of the sixties, hence the title of the BBC comedy show "Ab Fab", and "Austin Powers" use of the word.