Definitions for jackstayˈdʒækˌsteɪ
Random House Webster's College Dictionary
a rod or batten, following a yard, gaff, or boom, to which one edge of a sail is bent.
a rail for guiding the movement of the hanks of a sail.
Origin of jackstay:
A stay (rope, bar or batten), running along a ship's yard, to which is attached the head of a square sail.
a rail of wood or iron stretching along a yard of a vessel, to which the sails are fastened