The Collaborative International Dictionary of English v.0.48:

Stirrup \Stir"rup\, n. [OE. stirop, AS. stigr[=a]p; st[imac]gan
   to mount, ascend + r[=a]p a rope; akin to G. stegreif a
   stirrup. [root]164. See Sty, v. i., and Rope.]
   1. A kind of ring, or bent piece of metal, wood, leather, or
      the like, horizontal in one part for receiving the foot of
      a rider, and attached by a strap to the saddle, -- used to
      assist a person in mounting a horse, and to enable him to
      sit steadily in riding, as well as to relieve him by
      supporting a part of the weight of the body.
      [1913 Webster]

            Our host upon his stirpoes stood anon. --Chaucer.
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   2. (Carp. & Mach.) Any piece resembling in shape the stirrup
      of a saddle, and used as a support, clamp, etc. See
      Bridle iron.
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   3. (Naut.) A rope secured to a yard, with a thimble in its
      lower end for supporting a footrope. --Totten.
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   Stirrup bone (Anat.), the stapes.

   Stirrup cup, a parting cup taken after mounting.

   Stirrup iron, an iron stirrup.

   Stirrup leather, or Stirrup strap, the strap which
      attaches a stirrup to the saddle. See Stirrup, 1.
      [1913 Webster]

WordNet (r) 3.0 (2006):

stirrup iron
    n 1: support consisting of metal loops into which rider's feet
         go [syn: stirrup, stirrup iron]