The Collaborative International Dictionary of English v.0.48:

Backpiece \Back"piece`\, Backplate \Back"plate`\, n. [Back, n.
   or a. + piece, plate. ]
   1. A piece, or plate which forms the back of anything, or
      which covers the back.
      [1913 Webster]

   2. Specifically: A piece of plate armor protecting the back;
      -- it is worn as part of a cuirass.
      [WordNet 1.5] Backrack

WordNet (r) 3.0 (2006):

backplate
    n 1: plate armor protecting the back; worn as part of a cuirass