The Collaborative International Dictionary of English v.0.48:

Buckler \Buc"kler\, n. [OE. bocler, OF. bocler, F. bouclier, a
   shield with a boss, from OF. bocle, boucle, boss. See
   Buckle, n.]
   1. A kind of shield, of various shapes and sizes, worn on one
      of the arms (usually the left) for protecting the front of
      the body.
      [1913 Webster]

   Note: In the sword and buckler play of the Middle Ages in
         England, the buckler was a small shield, used, not to
         cover the body, but to stop or parry blows.
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   2. (Zool.)
      (a) One of the large, bony, external plates found on many
          ganoid fishes.
      (b) The anterior segment of the shell of trilobites.
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   3. (Naut.) A block of wood or plate of iron made to fit a
      hawse hole, or the circular opening in a half-port, to
      prevent water from entering when the vessel pitches.
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   Blind buckler (Naut.), a solid buckler.

   Buckler mustard (Bot.), a genus of plants (Biscutella)
      with small bright yellow flowers. The seed vessel on
      bursting resembles two bucklers or shields.

   Buckler thorn, a plant with seed vessels shaped like a
      buckler. See Christ's thorn.

   Riding buckler (Naut.), a buckler with a hole for the
      passage of a cable.
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WordNet (r) 3.0 (2006):

buckler mustard
    n 1: plant of southeastern Europe having yellow flowers like
         those of mustard and pods with open valves resembling
         bucklers [syn: buckler mustard, Biscutalla laevigata]