Definitions for "Marline spike"
A pointed iron pin about 16 inches long, furnished with a round head, used by riggers and seamen to separate the strands of rope when splicing and also as a lever when putting on seizing, marling etc.
A tapering, sharp-pointed, iron pin used in separating the strands of a rope for splicing.
A pointed metal spike used by deck crew to separate strands of rope for splicing or untying knots. Something done in a skillful manner is called "marline spike seamanship."