F.A.B. interj Brit an expression of agreement to, or acknowledgment of, a command [C20: from British television series Thunderbirds]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014