2003 Archive 
...absolutely nothing.
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".