The Collaborative International Dictionary of English v.0.48:

Curb \Curb\, n.
   1. That which curbs, restrains, or subdues; a check or
      hindrance; esp., a chain or strap attached to the upper
      part of the branches of a bit, and capable of being drawn
      tightly against the lower jaw of the horse.
      [1913 Webster]

            He that before ran in the pastures wild
            Felt the stiff curb control his angry jaws.
                                                  --Drayton.
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            By these men, religion,that should be
            The curb, is made the spur of tyranny. --Denham.
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   2. (Arch.) An assemblage of three or more pieces of timber,
      or a metal member, forming a frame around an opening, and
      serving to maintain the integrity of that opening; also, a
      ring of stone serving a similar purpose, as at the eye of
      a dome.
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   3. A frame or wall round the mouth of a well; also, a frame
      within a well to prevent the earth caving in.
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   4. A curbstone.
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   5. (Far.) A swelling on the back part of the hind leg of a
      horse, just behind the lowest part of the hock joint,
      generally causing lameness. --James Law.
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   Curb bit, a stiff bit having branches by which a leverage
      is obtained upon the jaws of horse. --Knight.

   Curb pins (Horology), the pins on the regulator which
      restrain the hairspring.

   Curb plate (Arch.), a plate serving the purpose of a curb.


   Deck curb. See under Deck.
      [1913 Webster]

WordNet (r) 3.0 (2006):

curb bit
    n 1: a horse's bit with an attached chain or strap to check the
         horse [syn: curb, curb bit]