Source : Webster's Revised Unabridged Dictionary (1913)

Forestay \Fore"stay`\, n. (Naut.)
   A large, strong rope, reaching from the foremast head to the
   bowsprit, to support the mast. See Illust. under {Ship}.

Source : WordNet®

forestay
     n : an adjustable stay from the foremast to the deck or
         bowsprit; controls the bending of the mast