Source : Webster's Revised Unabridged Dictionary (1913)

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

   2. Destitute of a chief or leader. --Sir W. Raleigh.

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

Source : WordNet®

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}]