Source : Webster's Revised Unabridged Dictionary (1913)

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.

   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.

   2. (Zo["o]l.)
      (a) One of the large, bony, external plates found on many
          ganoid fishes.
      (b) The anterior segment of the shell of trilobites.

   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.

   {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.

Source : WordNet®

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