The Collaborative International Dictionary of English v.0.48:

Breech \Breech\ (br[=e]ch or br[i^]ch; 277), n. [See
   Breeches.]
   1. The lower part of the body behind; the buttocks.
      [1913 Webster]

   2. Breeches. [Obs.] --Shak.
      [1913 Webster]

   3. The hinder part of anything; esp., the part of a cannon,
      or other firearm, behind the chamber.
      [1913 Webster]

   4. (Naut.) The external angle of knee timber, the inside of
      which is called the throat.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Breech \Breech\, v. t. [imp. & p. p. Breeched; p. pr. & vb. n.
   Breeching.]
   1. To put into, or clothe with, breeches.
      [1913 Webster]

            A great man . . . anxious to know whether the
            blacksmith's youngest boy was breeched. --Macaulay.
      [1913 Webster]

   2. To cover as with breeches. [Poetic]
      [1913 Webster]

            Their daggers unmannerly breeched with gore. --Shak.
      [1913 Webster]

   3. To fit or furnish with a breech; as, to breech a gun.
      [1913 Webster]

   4. To whip on the breech. [Obs.]
      [1913 Webster]

            Had not a courteous serving man conveyed me away,
            whilst he went to fetch whips, I think, in my
            conscience, he would have breeched me. --Old Play.
      [1913 Webster]

   5. To fasten with breeching.
      [1913 Webster]

WordNet (r) 3.0 (2006):

breech
    n 1: opening in the rear of the barrel of a gun where bullets
         can be loaded [syn: breech, rear of barrel, rear of
         tube]

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "breech":
   afterpart, afterpiece, back, back door, back seat, back side,
   backside, bed, bedrock, behind, belly, bottom, bottom side,
   buttocks, derriere, downside, fanny, fundament, hardpan, heel,
   hind end, hind part, hindhead, lower side, lowest layer,
   lowest level, nether side, nethermost level, occiput, posterior,
   postern, rear, rear end, rearward, reverse, rock bottom, rump,
   stern, substratum, tail, tail end, tailpiece, underbelly,
   underlayer, underneath, underside