The Collaborative International Dictionary of English v.0.48:

Headless \Head"less\, a. [AS. he['a]fodle['a]s.]
   1. Having no head; beheaded; as, a headless body, neck, or
      carcass.
      [1913 Webster]

   2. Destitute of a chief or leader. --Sir W. Raleigh.
      [1913 Webster]

   3. Destitute of understanding or prudence; foolish; rash;
      obstinate; mindless. [Obs.]
      [1913 Webster]

            Witless headiness in judging or headless hardiness
            in condemning.                        --Spenser.
      [1913 Webster]

WordNet (r) 3.0 (2006):

headless
    adj 1: not having a head or formed without a head ; "the
           headless horseman"; "brads are headless nails" [ant:
           headed]
    2: not using intelligence [syn: brainless, headless]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "headless":
   brainless, crownless, foolish, ignorant, inept, insensate,
   irrational, lackbrained, lean-minded, lean-witted, mindless,
   not bright, of little brain, pea-brained, pin-brained, reasonless,
   senseless, slackminded, slackwitted, topless, ungifted,
   unintellectual, unintelligent, unreasoning, untalented, unthinking,
   unwise, witless