The Collaborative International Dictionary of English v.0.48:

Hammer \Ham"mer\ (h[a^]m"m[~e]r), n. [OE. hamer, AS. hamer,
   hamor; akin to D. hamer, G. & Dan. hammer, Sw. hammare, Icel.
   hamarr, hammer, crag, and perh. to Gr. 'a`kmwn anvil, Skr.
   a[,c]man stone.]
   1. An instrument for driving nails, beating metals, and the
      like, consisting of a head, usually of steel or iron,
      fixed crosswise to a handle.
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            With busy hammers closing rivets up.  --Shak.
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   2. Something which in form or action resembles the common
      hammer; as:
      (a) That part of a clock which strikes upon the bell to
          indicate the hour.
      (b) The padded mallet of a piano, which strikes the wires,
          to produce the tones.
      (c) (Anat.) The malleus. See under Ear.
      (d) (Gun.) That part of a gunlock which strikes the
          percussion cap, or firing pin; the cock; formerly,
          however, a piece of steel covering the pan of a
          flintlock musket and struck by the flint of the cock
          to ignite the priming.
      (e) Also, a person or thing that smites or shatters; as,
          St. Augustine was the hammer of heresies.
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                He met the stern legionaries [of Rome] who had
                been the "massive iron hammers" of the whole
                earth.                            --J. H.
                                                  Newman.
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   3. (Athletics) A spherical weight attached to a flexible
      handle and hurled from a mark or ring. The weight of head
      and handle is usually not less than 16 pounds.
      [Webster 1913 Suppl.]

   Atmospheric hammer, a dead-stroke hammer in which the
      spring is formed by confined air.

   Drop hammer, Face hammer, etc. See under Drop, Face,
      etc.

   Hammer fish. See Hammerhead.

   Hammer hardening, the process of hardening metal by
      hammering it when cold.

   Hammer shell (Zool.), any species of Malleus, a genus of
      marine bivalve shells, allied to the pearl oysters, having
      the wings narrow and elongated, so as to give them a
      hammer-shaped outline; -- called also hammer oyster.

   To bring to the hammer, to put up at auction.
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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.
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   2. (Zool.) A fresh-water fish; the stone-roller.
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   3. (Zool.) An African fruit bat (Hypsignathus monstrosus);
      -- so called from its large blunt nozzle.
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