The Collaborative International Dictionary of English v.0.48:

Skull \Skull\, n. [OE. skulle, sculle, scolle; akin to Scot.
   skull, skoll, a bowl, Sw. skalle skull, skal a shell, and E.
   scale; cf. G. hirnschale, Dan. hierneskal. Cf. Scale of a
   balance.]
   1. (Anat.) The skeleton of the head of a vertebrate animal,
      including the brain case, or cranium, and the bones and
      cartilages of the face and mouth. See Illusts. of
      Carnivora, of Facial angles under Facial, and of
      Skeleton, in Appendix.
      [1913 Webster]

   Note: In many fishes the skull is almost wholly cartilaginous
         but in the higher vertebrates it is more or less
         completely ossified, several bones are developed in the
         face, and the cranium is made up, wholly or partially,
         of bony plates arranged in three segments, the frontal,
         parietal, and occipital, and usually closely united in
         the adult.
         [1913 Webster]

   2. The head or brain; the seat of intelligence; mind.
      [1913 Webster]

            Skulls that can not teach, and will not learn.
                                                  --Cowper.
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   3. A covering for the head; a skullcap. [Obs. & R.]
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            Let me put on my skull first.         --Beau. & Fl.
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   4. A sort of oar. See Scull.
      [1913 Webster]

   Skull and crossbones, a symbol of death. See Crossbones.
      [1913 Webster]

WordNet (r) 3.0 (2006):

skull and crossbones
    n 1: emblem warning of danger or death

Moby Thesaurus II by Grady Ward, 1.0:

90 Moby Thesaurus words for "skull and crossbones":
   Azrael, Black Death, Death, Grim Reaper, Pale Death, Reaper,
   angel of death, armory, badge, badge of office, badges, baton,
   blazonry, brassard, button, cap and gown, chain, chain of office,
   class ring, cockade, collar, cross, crossbones, danger sign,
   decoration, dress, eagle, early symptom, emblems, ensigns,
   falling barometer, fasces, figurehead, fleur-de-lis,
   gathering clouds, hammer and sickle, heraldry, high sign, insignia,
   lapel pin, livery, mace, mantle, markings, medal, memento mori,
   mortarboard, old school tie, omen, pale horse, pale rider, pin,
   precursor, preliminary sign, premonitory sign, premonitory symptom,
   prodroma, prodrome, quarantine flag, red flag, red light, regalia,
   ring, rose, school ring, shamrock, sickle of Death, sigillography,
   skull, sphragistics, staff, storm petrel, stormy petrel, swastika,
   symptom, tartan, that fell sergeant, that grim ferryman, thistle,
   thundercloud, thunderhead, tie, uniform, verge, wand, warning sign,
   warning signal, white cross, yellow flag, yellow jack