The Collaborative International Dictionary of English v.0.48:

Bur \Bur\, Burr \Burr\ (b[^u]r), n. [OE. burre burdock; cf. Dan.
   borre, OSw. borra, burdock, thistle; perh. akin to E. bristle
   (burr- for burz-), or perh. to F. bourre hair, wool, stuff;
   also, according to Cotgrave, "the downe, or hairie coat,
   wherewith divers herbes, fruits, and flowers, are covered,"
   fr. L. burrae trifles, LL. reburrus rough.]
   1. (Bot.) Any rough or prickly envelope of the seeds of
      plants, whether a pericarp, a persistent calyx, or an
      involucre, as of the chestnut and burdock; a seed vessel
      having hooks or prickles. Also, any weed which bears burs.
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            Amongst rude burs and thistles.       --Milton.
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            Bur and brake and brier.              --Tennyson.
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   2. The thin ridge left by a tool in cutting or shaping metal.
      See Burr, n., 2.
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   3. A ring of iron on a lance or spear. See Burr, n., 4.
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   4. The lobe of the ear. See Burr, n., 5.
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   5. The sweetbread.
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   6. A clinker; a partially vitrified brick.
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   7. (Mech.)
      (a) A small circular saw.
      (b) A triangular chisel.
      (c) A drill with a serrated head larger than the shank; --
          especially a small drill bit used by dentists.
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   8. [Cf. Gael. borr, borra, a knob, bunch.] (Zool.) The round
      knob of an antler next to a deer's head. [Commonly written
      burr.]
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   Bur oak (Bot.), a useful and ornamental species of oak
      (Quercus macrocarpa) with ovoid acorns inclosed in deep
      cups imbricated with pointed scales. It grows in the
      Middle and Western United States, and its wood is tough,
      close-grained, and durable.

   Bur reed (Bot.), a plant of the genus Sparganium, having
      long ribbonlike leaves.
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WordNet (r) 3.0 (2006):

bur
    n 1: seed vessel having hooks or prickles [syn: bur, burr]
    2: small bit used in dentistry or surgery [syn: bur, burr]
    v 1: remove the burrs from [syn: bur, burr]