Source : Webster's Revised Unabridged Dictionary (1913)

Hammerhead \Ham"mer*head`\, n.
   1. (Zo["o]l.) A shark of the genus {Sphyrna} or {Zyg[ae]na},
      having the eyes set on projections from the sides of the
      head, which gives it a hammer shape. The {Sphyrna
      zyg[ae]na} is found in the North Atlantic. Called also
      {hammer fish}, and {balance fish}.

Source : WordNet®

hammerhead
     n 1: these words are used to express a low opinion of someone's
          intelligence [syn: {dunce}, {dunderhead}, {numskull}, {blockhead},
           {bonehead}, {lunkhead}, {knucklehead}, {loggerhead}, {muttonhead},
           {shithead}, {fuckhead}]
     2: the striking part of a hammer
     3: medium-sized live-bearing shark with eyes at either end of a
        flattened hammer-shaped head; worldwide in warm waters;
        can be dangerous [syn: {hammerhead shark}]