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.
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   2. An outside piece taken from a log or timber in sawing it
      into boards, planks, etc.
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   3. (Zool.) The wryneck. [Prov. Eng.]
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   4. (Naut.) The slack part of a sail.
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   Slab line (Naut.), a line or small rope by which seamen
      haul up the foot of the mainsail or foresail. --Totten.
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