The Collaborative International Dictionary of English v.0.48:

Shield \Shield\, n. [OE. sheld, scheld, AS. scield, scild,
   sceld, scyld; akin to OS. scild, OFries. skeld, D. & G.
   schild, OHG. scilt, Icel. skj["o]ldr, Sw. sk["o]ld, Dan.
   skiold, Goth. skildus; of uncertain origin. Cf. Sheldrake.]
   1. A broad piece of defensive armor, carried on the arm, --
      formerly in general use in war, for the protection of the
      body. See Buckler.
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            Now put your shields before your hearts and fight,
            With hearts more proof than shields.  --Shak.
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   2. Anything which protects or defends; defense; shelter;
      protection. "My council is my shield." --Shak.
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   3. Figuratively, one who protects or defends.
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            Fear not, Abram; I am thy shield, and thy exceeding
            great reward.                         --Gen. xv. 1.
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   4. (Bot.) In lichens, a Hardened cup or disk surrounded by a
      rim and containing the fructification, or asci.
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   5. (Her.) The escutcheon or field on which are placed the
      bearings in coats of arms. Cf. Lozenge. See Illust. of
      Escutcheon.
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   6. (Mining & Tunneling) A framework used to protect workmen
      in making an adit under ground, and capable of being
      pushed along as excavation progresses.
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   7. A spot resembling, or having the form of, a shield.
      "Bespotted as with shields of red and black." --Spenser.
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   8. A coin, the old French crown, or ['e]cu, having on one
      side the figure of a shield. [Obs.] --Chaucer.
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   Shield fern (Bot.), any fern of the genus Aspidium, in
      which the fructifications are covered with shield-shaped
      indusia; -- called also wood fern. See Illust. of
      Indusium.
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WordNet (r) 3.0 (2006):

shield fern
    n 1: any of various ferns of the genera Dryopteris or
         Polystichum or Lastreopsis having somewhat shield-shaped
         coverings on the sori [syn: shield fern, buckler fern]