Definition of jackline in English:

noun

• 1Any of various kinds of (thin) rope or line used for a particular purpose, (now) especially in sailing.

• 2Nautical
  A rope, wire, etc., running along the length of a ship's deck and secured at both ends, to which a crew member's safety harness is clipped so that he or she can move about the deck safely in rough weather, conditions of poor visibility, etc.