Source : Webster's Revised Unabridged Dictionary (1913)

Backstay \Back"stay`\, n. [Back, a. orn.+ stay.]
   1. (Naut.) A rope or stay extending from the masthead to the
      side of a ship, slanting a little aft, to assist the
      shrouds in supporting the mast. [ Often used in the
      plural.]

   2. A rope or strap used to prevent excessive forward motion.

Source : WordNet®

backstay
     n : a stay that supports the back of something