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]