The Collaborative International Dictionary of English v.0.48:

Fabulous \Fab"u*lous\ (f[a^]b"[-u]*l[u^]s), a. [L. fabulosus;
   cf. F. fabuleux. See Fable.]
   1. Feigned, as a story or fable; related in fable; devised;
      invented; not real; fictitious; as, a fabulous
      description; a fabulous hero.
      [1913 Webster]

            The fabulous birth of Minerva.        --Chesterfield.
      [1913 Webster]

   2. Beyond belief; exceedingly great; as, a fabulous price.
      --Macaulay.
      [1913 Webster]

   3. Exceptionally good; extremely pleasing. [informal]
      [PJC]

   Fabulous age, that period in the history of a nation of
      which the only accounts are myths and unverified legends;
      as, the fabulous age of Greece and Rome. --
      Fab"u*lous*ly, adv. -- Fab"u*lous*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fabulous
    adj 1: extremely pleasing; "a fabulous vacation" [syn:
           fabulous, fab]
    2: based on or told of in traditional stories; lacking factual
       basis or historical validity; "mythical centaurs"; "the
       fabulous unicorn" [syn: fabulous, mythic, mythical,
       mythologic, mythological]
    3: barely credible; "the fabulous endurance of a marathon
       runner"

Moby Thesaurus II by Grady Ward, 1.0:

149 Moby Thesaurus words for "fabulous":
   a bit much, abandoned, ace, allegoric, amazing, appalling,
   astonishing, astounding, beguiling, bewildering, boundless,
   celebrated, concocted, conspicuous, cooked-up, cool, copacetic,
   egregious, enigmatic, enormous, exaggerated, exceptional,
   excessive, exorbitant, extraordinary, extravagant, extreme, fab,
   fabled, fabricated, fancied, fanciful, fancy, fantasied, fantastic,
   fascinating, fictional, fictitious, fictive, figmental, forged,
   formidable, gigantic, gluttonous, great, groovy, hatched, high,
   hot, howling, hyperbolic, hypertrophied, imaginary, immoderate,
   incomprehensible, inconceivable, incontinent, incredible,
   indescribable, ineffable, inordinate, intemperate, invented, keen,
   legendary, made-up, magic, make-believe, manufactured, marked,
   marvelous, miraculous, monstrous, mythic, mythical, mythicized,
   mythified, mythological, mythopoeic, mythopoetic, neat, nonactual,
   nonfactual, nonrealistic, notable, noteworthy, noticeable, of mark,
   out of bounds, out of sight, outlandish, outrageous, outstanding,
   overbig, overdeveloped, overgreat, overgrown, overlarge, overmuch,
   overweening, parabolic, passing strange, phenomenal, portentous,
   preposterous, prodigious, put-up, puzzling, rare, rattling,
   remarkable, romantic, romanticized, sensational, signal, smashing,
   steep, stiff, storied, strange, striking, stupefying, stupendous,
   super, superb, superior, terrific, thumping, thundering, too much,
   trumped-up, unbelievable, unbridled, uncommon, unconscionable,
   undue, unexampled, unheard-of, unimaginable, unique, unparalleled,
   unprecedented, unreal, unreasonable, unrestrained, unspeakable,
   whopping, wonderful, wondrous