The Collaborative International Dictionary of English v.0.48:
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. [1913 Webster]WordNet (r) 3.0 (2006):
forestay n 1: an adjustable stay from the foremast to the deck or bowsprit; controls the bending of the mast