noun

• (obsolete) A shield resembling the Roman scutum. In modern usage, a smaller variety of shield is usually implied by this term.
  "The target or buckler was carried by the heavy armed foot, it answered to the scutum of the Romans; its form was sometimes that of a rectangular parallelogram, but more commonly had it's bottom rounded off; it was generally convex, being curved in it's breadth."
• (zoology) One of the large, bony, external plates found on many ganoid fishes.
  "Most zeids have spiny projections or bucklers at the dorsal- and anal-fin bases and bony plates on the ventral surface of the abdomen."
• (zoology) The anterior segment of the shell of a trilobite.
• (nautical) 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.

verb

• (transitive, obsolete) To shield; to defend.
  "Can Oxford, that did ever fence the right, / Now buckler falsehood with a pedigree?"