The Collaborative International Dictionary of English v.0.48:

Backstay \Back"stay`\ (b[a^]k"st[=a]`), n. [Back, a. or n. +
   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.]
      [1913 Webster]

   2. A rope or strap used to prevent excessive forward motion.
      [1913 Webster]

WordNet (r) 3.0 (2006):

backstay
    n 1: a stay that supports the back of something