The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. An outside piece taken from a log or timber in sawing it
      into boards, planks, etc.
      [1913 Webster]

   3. (Zool.) The wryneck. [Prov. Eng.]
      [1913 Webster]

   4. (Naut.) The slack part of a sail.
      [1913 Webster]

   Slab line (Naut.), a line or small rope by which seamen
      haul up the foot of the mainsail or foresail. --Totten.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Slab \Slab\, a. [Cf. Gael. & Ir. slaib mud, mire left on a river
   strand, and E. slop puddle.]
   Thick; viscous. [Obs.]
   [1913 Webster]

         Make the gruel thick and slab.           --Shak.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Slab \Slab\, n.
   That which is slimy or viscous; moist earth; mud; also, a
   puddle. [Obs.] --Evelyn.
   [1913 Webster]

WordNet (r) 3.0 (2006):

slab
    n 1: block consisting of a thick piece of something

Moby Thesaurus II by Grady Ward, 1.0:

106 Moby Thesaurus words for "slab":
   bar, beam, billet, board, boarding, chunk, clapboard, coat,
   coating, collop, cord, cordwood, covering, cut, deal, disk,
   driftwood, feuille, film, firewood, flap, foil, fold, glop, gunk,
   hardwood, hunk, ingot, lamella, lamina, laminated glass,
   laminated wood, lap, lath, lathing, lathwork, leaf, log, lumber,
   lump, membrane, mire, muck, ooze, pane, panel, panelboard,
   paneling, panelwork, patina, peel, pellicle, piece, plait, plank,
   planking, plate, plating, ply, plyboard, plywood, pole, post,
   puncheon, rasher, rod, safety glass, scum, shake, sheathing,
   sheathing board, sheet, sheeting, shingle, sideboard, siding, skin,
   slat, slice, slime, slob, slop, slosh, sludge, slum, slush,
   softwood, splat, splosh, stave, stick, stick of wood, stovewood,
   strip, table, tablet, three-by-four, timber, timbering, timberwork,
   two-by-four, veneer, wafer, weatherboard, wedge, wood



The Jargon File (version 4.4.7, 29 Dec 2003):

slab


    1. n. A continuous horizontal line of pixels, all with the same color.

    2. vi. To paint a slab on an output device. Apple's QuickDraw, like most
    other professional-level graphics systems, renders polygons and lines not
    with Bresenham's algorithm, but by calculating slab points for each scan
    line on the screen in succession, and then slabbing in the actual image
    pixels.