The Collaborative International Dictionary of English v.0.48:

Collar \Col"lar\, n. [OE. coler, coller, OF. colier, F. collier,
   necklace, collar, fr. OF. col neck, F. cou, fr. L. collum;
   akin to AS. heals, G. & Goth. hals. Cf. Hals, n.]
   1. Something worn round the neck, whether for use, ornament,
      restraint, or identification; as, the collar of a coat; a
      lady's collar; the collar of a dog.
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   2. (Arch.)
      (a) A ring or cincture.
      (b) A collar beam.
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   3. (Bot.) The neck or line of junction between the root of a
      plant and its stem. --Gray.
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   4. An ornament worn round the neck by knights, having on it
      devices to designate their rank or order.
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   5. (Zool.)
      (a) A ringlike part of a mollusk in connection with
          esophagus.
      (b) A colored ring round the neck of a bird or mammal.
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   6. (Mech.) A ring or round flange upon, surrounding, or
      against an object, and used for restraining motion within
      given limits, or for holding something to its place, or
      for hiding an opening around an object; as, a collar on a
      shaft, used to prevent endwise motion of the shaft; a
      collar surrounding a stovepipe at the place where it
      enters a wall. The flanges of a piston and the gland of a
      stuffing box are sometimes called collars.
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   7. (Naut.) An eye formed in the bight or bend of a shroud or
      stay to go over the masthead; also, a rope to which
      certain parts of rigging, as dead-eyes, are secured.
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   8. (Mining) A curb, or a horizontal timbering, around the
      mouth of a shaft. --Raymond.
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   Collar beam (Arch.), a horizontal piece of timber
      connecting and tying together two opposite rafters; --
      also, called simply collar.

   Collar of brawn, the quantity of brawn bound up in one
      parcel. [Eng.] --Johnson.

   Collar day, a day of great ceremony at the English court,
      when persons, who are dignitaries of honorary orders, wear
      the collars of those orders.

   To slip the collar, to get free; to disentangle one's self
      from difficulty, labor, or engagement. --Spenser.
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