The Collaborative International Dictionary of English v.0.48:

Casemate \Case"mate\, n. [F. casemate, fr. It. casamatta, prob.
   from casa house + matto, f. matta, mad, weak, feeble, dim.
   from the same source as E. -mate in checkmate.]
   [1913 Webster]
   1. (Fort.) A bombproof chamber, usually of masonry, in which
      cannon may be placed, to be fired through embrasures; or
      one capable of being used as a magazine, or for quartering
      troops.
      [1913 Webster]

   2. (Arch.) A hollow molding, chiefly in cornices.
      [1913 Webster]