What does FAB stand for, as in 'Thunderbirds'?

FAB does not stand for anything, it is a shortened version of 'fabulous'.

http://www.thunderbirdsonline.co.uk/

The official version of the origin of FAB (from Gerry Anderson himself) is
: that the term "FAB" refers to the use of "Fab" as a sixties thing, such as
: the Beatles, "the Fab Four".
: Anderson repeatedly said that FAB had no meaning, fab == a contraction of
: fabulous was in _very_ widespread use at the time.

https://groups.google.com/forum/#!msg/rec.arts.tv/n2WHs187FNk/_o2-n1nEvHMJ

The definitive answer came from Sylvia Anderson herself during a promotional tour of her book. 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".

http://www.imdb.com/title/tt0057790/faq

The characters use the radio sign-off "F. A. B." rather than "Roger" or "Over and out". Anderson was often asked what F. A. B. stood for, but in fact it simply stood for "fab" (short for "fabulous"), which was a 1960s catchphrase. Later this was unofficially decided to mean Fully Advised and Briefed, following on from P-W-O-R (Proceeding With Orders Received), a similar radio confirmation code in the series Stingray. In Anderson's next series, Captain Scarlet and the Mysterons, S.I.G. (Spectrum is Green) would continue the tradition of initialled call signs.

"FAB" was used by radio operators in WW2 and is not short for "fabulous" in this context. The acronym means 'Fully Acknowledged Broadcast" and was called as "F" "A" "B" - not FAB as a word.

(Editor: the problem with this theory is that the author has stated that it had no meaning.)