What is "slab line"

Slab \Slab\, n. [OE. slabbe, of uncertain origin; perhaps originally meaning, a smooth piece, and akin to slape, Icel. sleipr slippery, and E. slip, v. i.]

1. A thin piece of anything, especially of marble or other stone, having plane surfaces.
   --Gwilt.

2. An outside piece taken from a log or timber in sawing it into boards, planks, etc.

3. (Zo["o]l.) The wryneck. [Prov. Eng.]

4. (Naut.) The slack part of a sail.

   Slab line (Naut.), a line or small rope by which seamen haul up the foot of the mainsail or foresail.
   --Totten.

n. (context nautical English) A line or small rope by which sailors haul up the foot of the mainsail or foresail.