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"