Source : Webster's Revised Unabridged Dictionary (1913)

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.

Slab \Slab\, a. [Cf. Gael. & Ir. slaib mud, mire left on a river
   strand, and E. slop puddle.]
   Thick; viscous. [Obs.]

         Make the gruel thick and slab.           --Shak.

Slab \Slab\, n.
   That which is slimy or viscous; moist earth; mud; also, a
   puddle. [Obs.] --Evelyn.

Source : WordNet®

slab
     n : block consisting of a thick piece of something
     [also: {slabbing}, {slabbed}]