The Collaborative International Dictionary of English v.0.48:

Unbreech \Un*breech"\, v. t. [imp. & p. p. Unbreeched; p. pr.
   & vb. n. Unbreeching.] [1st pref. un- + breech.]
   1. To remove the breeches of; to divest or strip of breeches.
      --Shak.
      [1913 Webster]

   2. (Gun.) To free the breech of, as a cannon, from its
      fastenings or coverings. --Pennant.
      [1913 Webster]