The Collaborative International Dictionary of English v.0.48:

Brainless \Brain"less\, a.
   Without understanding; silly; thoughtless; witless. --
   Brain"less*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

brainless
    adj 1: not using intelligence [syn: brainless, headless]

Moby Thesaurus II by Grady Ward, 1.0:

93 Moby Thesaurus words for "brainless":
   apish, asinine, batty, befooled, beguiled, besotted, buffoonish,
   cockeyed, crazy, credulous, daffy, daft, dazed, dizzy, doting,
   dumb, empty-headed, fatuitous, fatuous, featherbrained,
   featherheaded, flaky, fluttery, fond, fool, foolheaded, foolish,
   frivolous, fuddled, futile, gaga, giddy, giddy-brained,
   giddy-headed, giddy-pated, giddy-witted, goofy, gulled, harebrain,
   harebrained, headless, idiotic, ignorant, imbecile, inane, inept,
   infatuated, insane, insensate, irrational, kooky, lackbrained,
   lean-minded, lean-witted, loony, mad, maudlin, mindless, moronic,
   nitwitted, not bright, nutty, of little brain, pea-brained,
   pin-brained, rattlebrained, rattleheaded, rattlepated, reasonless,
   sappy, scatterbrained, scramblebrained, screwy, senseless,
   sentimental, shatterbrained, silly, slackminded, slackwitted,
   stupid, thoughtless, ungifted, unintellectual, unintelligent,
   unreasoning, untalented, unthinking, unwise, unwitty, wacky,
   weak-minded, wet, witless