The Collaborative International Dictionary of English v.0.48:

Asinine \As"i*nine\, a. [L. asininus, fr. asinus ass. See
   Ass.]
   Of or belonging to, or having the qualities of, the ass, as
   stupidity and obstinacy. "Asinine nature." --B. Jonson.
   "Asinine feast." --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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