Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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