From WordNet (r) 2.0 (August 2003) [wn]:
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]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]