From 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]
From 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]
From 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]