The Collaborative International Dictionary of English v.0.48:

Spavin \Spav"in\, n. [OE. spaveyne, OF. esparvain, F.
   ['e]parvin; akin to OF. espervier a sparrow hawk, F.
   ['e]pervier, fr. OHG. sparw[=a]ri (G. sperber), fr. OHG.
   sparo sparrow, because this disease makes the horse raise the
   infirm leg in the manner of a sparrow hawk or sparrow. See
   Sparrow.] (Far.)
   A disease of horses characterized by a bony swelling
   developed on the hock as the result of inflammation of the
   bones; also, the swelling itself. The resulting lameness is
   due to the inflammation, and not the bony tumor as popularly
   supposed. --Harbaugh.
   [1913 Webster]

   Bog spavin, a soft swelling produced by distention of the
      capsular ligament of the hock; -- called also blood
      spavin.

   Bone spavin, spavin attended with exostosis; ordinary
      spavin.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

bog \bog\ (b[o^]g), n. [Ir. & Gael. bog soft, tender, moist: cf.
   Ir. bogach bog, moor, marsh, Gael. bogan quagmire.]
   [1913 Webster]
   1. A quagmire filled with decayed moss and other vegetable
      matter; wet spongy ground where a heavy body is apt to
      sink; a marsh; a morass.
      [1913 Webster]

            Appalled with thoughts of bog, or caverned pit,
            Of treacherous earth, subsiding where they tread.
                                                  --R. Jago.
      [1913 Webster]

   2. A little elevated spot or clump of earth, roots, and
      grass, in a marsh or swamp. [Local, U. S.]
      [1913 Webster]

   Bog bean. See Buck bean.

   Bog bumper (bump, to make a loud noise), Bog blitter,
   Bog bluiter, Bog jumper, the bittern. [Prov.]

   Bog butter, a hydrocarbon of butterlike consistence found
      in the peat bogs of Ireland.

   Bog earth (Min.), a soil composed for the most part of
      silex and partially decomposed vegetable fiber. --P. Cyc.

   Bog moss. (Bot.) Same as Sphagnum.

   Bog myrtle (Bot.), the sweet gale.

   Bog ore. (Min.)
      (a) An ore of iron found in boggy or swampy land; a
          variety of brown iron ore, or limonite.
      (b) Bog manganese, the hydrated peroxide of manganese.

   Bog rush (Bot.), any rush growing in bogs; saw grass.

   Bog spavin. See under Spavin.
      [1913 Webster]

WordNet (r) 3.0 (2006):

bog spavin
    n 1: spavin caused by collection of fluids