The Collaborative International Dictionary of English v.0.48:

Fatuous \Fat"u*ous\, a. [L. fatuus.]
   1. Feeble in mind; weak; silly; stupid; foolish; fatuitous.
      --Glanvill.
      [1913 Webster]

   2. Without reality; illusory, like the ignis fatuus.
      [1913 Webster]

            Thence fatuous fires and meteors take their birth.
                                                  --Danham.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fatuous
    adj 1: devoid of intelligence [syn: asinine, fatuous,
           inane, mindless, vacuous]

Moby Thesaurus II by Grady Ward, 1.0:

133 Moby Thesaurus words for "fatuous":
   absurd, air-built, airy, apish, asinine, barren, batty, befooled,
   beguiled, besotted, blank, blankminded, bootless, brainless,
   buffoonish, calm, catchpenny, chimerical, cloud-built, cockeyed,
   counterproductive, crazy, credulous, daffy, daft, dazed, dizzy,
   doting, dreamlike, dumb, effete, empty, empty-headed, ethereal,
   etiolated, fanciful, fatuitous, feckless, flaky, flimsy, fond,
   fool, foolheaded, foolish, fribble, fribbling, frivolous, frothy,
   fruitless, fuddled, futile, gaga, gaseous, goofy, gossamery,
   gulled, hollow, idiotic, idle, illusory, imaginary, imbecile,
   inadequate, inane, incogitant, ineffective, ineffectual,
   inefficacious, inept, infatuated, inoperative, insane, insensate,
   invalid, kooky, light, loony, mad, maudlin, moronic, nirvanic,
   nugacious, nugatory, nutty, oblivious, of no force, otiose,
   passive, phantomlike, quietistic, rarefied, relaxed, sappy, screwy,
   senseless, sentimental, shadowy, shallow, silly, slender, slight,
   spirituous, sterile, stupid, subtile, subtle, superficial, tenuous,
   thoughtfree, thoughtless, tranquil, trifling, trite, trivial,
   unavailing, unideaed, unintellectual, unoccupied, unreal,
   unreasoning, unthinking, unwitty, useless, vacant, vacuous, vain,
   vapid, vaporous, wacky, weak-minded, wet, windy, witless