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.
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   2. Breeches. [Obs.] --Shak.
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   3. The hinder part of anything; esp., the part of a cannon,
      or other firearm, behind the chamber.
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   4. (Naut.) The external angle of knee timber, the inside of
      which is called the throat.
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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.
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            A great man . . . anxious to know whether the
            blacksmith's youngest boy was breeched. --Macaulay.
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   2. To cover as with breeches. [Poetic]
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            Their daggers unmannerly breeched with gore. --Shak.
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   3. To fit or furnish with a breech; as, to breech a gun.
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   4. To whip on the breech. [Obs.]
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            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.
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   5. To fasten with breeching.
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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]