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.
      [1913 Webster]

   2. (Carp. & Mach.) Any piece resembling in shape the stirrup
      of a saddle, and used as a support, clamp, etc. See
      Bridle iron.
      [1913 Webster]

   3. (Naut.) A rope secured to a yard, with a thimble in its
      lower end for supporting a footrope. --Totten.
      [1913 Webster]

   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]

The Collaborative International Dictionary of English v.0.48:

Bridle iron \Bri"dle i`ron\ (Arch.)
   A strong flat bar of iron, so bent as to support, as in a
   stirrup, one end of a floor timber, etc., where no sufficient
   bearing can be had; -- called also stirrup and hanger.
   [1913 Webster]

WordNet (r) 3.0 (2006):

stirrup
    n 1: support consisting of metal loops into which rider's feet
         go [syn: stirrup, stirrup iron]
    2: the stirrup-shaped ossicle that transmits sound from the
       incus to the cochlea [syn: stapes, stirrup]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "stirrup":
   Eustachian tube, anvil, auditory apparatus, auditory canal,
   auditory meatus, auditory nerve, auditory ossicles, auditory tube,
   auricle, basilar membrane, bellyband, bony labyrinth,
   cauliflower ear, cinch, cochlea, conch, concha, drumhead, ear,
   ear lobe, eardrum, endolymph, external ear, girt, girth, hammer,
   horn, incus, inner ear, jockey, lobe, lobule, lug, malleus,
   mastoid process, middle ear, organ of Corti, outer ear,
   oval window, perilymph, pinna, pommel, round window,
   secondary eardrum, semicircular canals, shell, stapes, surcingle,
   tympanic cavity, tympanic membrane, tympanum, vestibule