FAB from the TV show "Thunderbirds" stood for "Full Acknowledgement of Broadcast." Gerry Anderson, the producer of "Thunderbirds" was known for these obscure catch phrases. In another series "Captain Scarlet" he usied SIG which stood for "Spectrum Is Green" They both meant roughly," ten-four", "I copy" and "roger that."
As a member of Fanderson (The Gerry Anderson Appreciation Society...yes, I have my anorak!!), I can tell you FAB doesn't stand for anything. In the 60's good things were 'fab', so Gerry just thought it would be a good 'catchphrase', and set up years of speculation!! In Capt Scarlet, both SIG and SIR were used (Spectrum is Green/Red) and even more obscure, Troy Tempest acknowledged orders in Stingray with PWOR (Proceeding with orders received!).