The Collaborative International Dictionary of English v.0.48:

Gelding \Geld"ing\, n. [Icel. gelding a gelding, akin to
   geldingr wether, eunuch, Sw. g[aum]lling gelding, Dan.
   gilding eunuch. See Geld, v. t.]
   A castrated animal; -- usually applied to a horse, but
   formerly used also of the human male.
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         They went down both into the water, Philip and the
         gelding, and Philip baptized him.        --Wyclif (Acts
                                                  viii. 38).
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The Collaborative International Dictionary of English v.0.48:

Gelding \Geld"ing\, p. pr., a., & vb. n.
   from Geld, v. t.
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The Collaborative International Dictionary of English v.0.48:

Geld \Geld\, v. t. [imp. & p. p. Gelded or Gelt (?); p. pr. &
   vb. n. Gelding.] [Icel. gelda to castrate; akin to Dan.
   gilde, Sw. g[aum]lla, and cf. AS. gilte a young sow, OHG.
   galt dry, not giving milk, G. gelt, Goth. gilpa siclke.]
   1. To castrate; to emasculate.
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   2. To deprive of anything essential.
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            Bereft and gelded of his patrimony.   --Shak.
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   3. To deprive of anything exceptionable; as, to geld a book,
      or a story; to expurgate. [Obs.] --Dryden.
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WordNet (r) 3.0 (2006):

gelding
    n 1: castrated male horse

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "gelding":
   Houyhnhnm, altering, blank cartridge, brood mare, castration,
   castrato, charger, colt, courser, critter, dobbin, dud,
   emasculation, entire, entire horse, equine, eunuch, filly, fixing,
   foal, horse, impotent, incompetent, invalid, mare, mount, nag,
   prancer, spaying, stallion, steed, stud, studhorse, tarpan,
   top horse, war-horse, weakling, wild horse