The Collaborative International Dictionary of English v.0.48:

Crab \Crab\ (kr[a^]b), n. [AS. crabba; akin to D. krab, G.
   krabbe, krebs, Icel. krabbi, Sw. krabba, Dan. krabbe, and
   perh. to E. cramp. Cf. Crawfish.]
   1. (Zool.) One of the brachyuran Crustacea. They are mostly
      marine, and usually have a broad, short body, covered with
      a strong shell or carapace. The abdomen is small and
      curled up beneath the body.
      [1913 Webster]

   Note: The name is applied to all the Brachyura, and to
         certain Anomura, as the hermit crabs. Formerly, it was
         sometimes applied to Crustacea in general. Many species
         are edible, the blue crab of the Atlantic coast being
         one of the most esteemed. The large European edible
         crab is Cancer padurus. Soft-shelled crabs are blue
         crabs that have recently cast their shells. See
         Cancer; also, Box crab, Fiddler crab, Hermit
         crab, Spider crab, etc., under Box, Fiddler.
         etc.
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   2. The zodiacal constellation Cancer.
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   3. [See Crab, a.] (Bot.) A crab apple; -- so named from its
      harsh taste.
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            When roasted crabs hiss in the bowl,
            Then nightly sings the staring owl.   --Shak.
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   4. A cudgel made of the wood of the crab tree; a crabstick.
      [Obs.] --Garrick.
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   5. (Mech.)
      (a) A movable winch or windlass with powerful gearing,
          used with derricks, etc.
      (b) A form of windlass, or geared capstan, for hauling
          ships into dock, etc.
      (c) A machine used in ropewalks to stretch the yarn.
      (d) A claw for anchoring a portable machine.
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   Calling crab. (Zool.) See Fiddler., n., 2.

   Crab apple, a small, sour apple, of several kinds; also,
      the tree which bears it; as, the European crab apple
      (Pyrus Malus var. sylvestris); the Siberian crab apple
      (Pyrus baccata); and the American (Pyrus coronaria).


   Crab grass. (Bot.)
      (a) A grass (Digitaria sanguinalis syn. Panicum
          sanguinalis); -- called also finger grass.
      (b) A grass of the genus Eleusine (Eleusine Indica);
          -- called also dog's-tail grass, wire grass, etc.


   Crab louse (Zool.), a species of louse (Phthirius pubis),
      sometimes infesting the human body.

   Crab plover (Zool.), an Asiatic plover (Dromas ardeola).


   Crab's eyes, or Crab's stones, masses of calcareous
      matter found, at certain seasons of the year, on either
      side of the stomach of the European crawfishes, and
      formerly used in medicine for absorbent and antacid
      purposes; the gastroliths.

   Crab spider (Zool.), one of a group of spiders
      (Laterigrad[ae]); -- called because they can run
      backwards or sideways like a crab.

   Crab tree, the tree that bears crab applies.

   Crab wood, a light cabinet wood obtained in Guiana, which
      takes a high polish. --McElrath.

   To catch a crab (Naut.), a phrase used of a rower:
      (a) when he fails to raise his oar clear of the water;
      (b) when he misses the water altogether in making a
          stroke.
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The Collaborative International Dictionary of English v.0.48:

Crab tree \Crab" tree\ (tr[=e]`).
   See under Crab.
   [1913 Webster]