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]

Moby Thesaurus II by Grady Ward, 1.0:

127 Moby Thesaurus words for "asinine":
   Boeotian, absurd, airy, apish, batty, beef-brained, beef-witted,
   befooled, beguiled, besotted, blockish, bovid, bovine, brainless,
   buffoonish, caprid, caprine, catchpenny, chumpish, cloddish,
   cockeyed, cowish, cowlike, crass, crazy, credulous, daffy, daft,
   dazed, deerlike, dense, dizzy, doltish, doting, dullard, dumb,
   duncical, duncish, empty, equestrian, equine, fat, fatuitous,
   fatuous, flaky, flimsy, fond, fool, foolheaded, foolish, fribble,
   fribbling, frivolous, frothy, fuddled, futile, gaga, goatish,
   goatlike, goofy, gross, gulled, hircine, hoggish, hoofed, horsy,
   idiotic, idle, imbecile, inane, ineducable, inept, infatuated,
   insane, irrational, klutzy, kooky, light, loony, lumpish, mad,
   maudlin, mindless, moronic, mulish, nugacious, nugatory, nutty,
   oafish, opaque, otiose, ovine, piggish, puerile, ruminant, sappy,
   screwy, senseless, sentimental, shallow, sheepish, sheeplike,
   silly, slender, slight, sottish, stupid, superficial, swinish,
   thick, thoughtless, trifling, trite, trivial, ungulate,
   unreasonable, unteachable, unwitty, vacuous, vain, vapid, wacky,
   weak-minded, wet, windy, witless, wrongheaded