[1] : The Collaborative International Dictionary of English v.0.48

backstay \back"stay`\ ba^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

[2] : WordNet (r) 2.0

backstay
     n : a stay that supports the back of something

Backstay >> B Words
Backstay, definition of term: Backstay
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