The Collaborative International Dictionary of English v.0.48:

fab \fab\ adj. [shortened form of fabulous.]
   extremely pleasing. [Colloq.]

   Syn: fabulous.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

fab
    adj 1: extremely pleasing; "a fabulous vacation" [syn:
           fabulous, fab]

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "fab":
   OK, ace-high, bad, bang-up, bonzer, boss, bully, but good, cool,
   corking, crackerjack, dandy, delicious, ducky, fine and dandy,
   gear, great, groovy, heavy, hot, hunky-dory, jam-up, just dandy,
   keen, marvy, mean, neat, nifty, nobby, okay, out of sight, peachy,
   peachy-keen, ripping, rum, scrumptious, slap-up, smashing, solid,
   something else, spiffing, spiffy, stunning, swell, tough, wizard