The Collaborative International Dictionary of English v.0.48:

Hammerhead \Ham"mer*head`\ (-h[e^]d`), n.
   1. (Zool.) 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.
      [1913 Webster]

   2. (Zool.) A fresh-water fish; the stone-roller.
      [1913 Webster]

   3. (Zool.) An African fruit bat (Hypsignathus monstrosus);
      -- so called from its large blunt nozzle.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hammerhead
    n 1: a stupid person; these words are used to express a low
         opinion of someone's intelligence [syn: dunce,
         dunderhead, numskull, blockhead, bonehead,
         lunkhead, hammerhead, knucklehead, loggerhead,
         muttonhead, shithead, dumbass, 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, hammerhead shark]