The Collaborative International Dictionary of English v.0.48:

Plank \Plank\, n. [OE. planke, OF. planque, planche, F. planche,
   fr. L. planca; cf. Gr. ?, ?, anything flat and broad. Cf.
   Planch.]
   1. A broad piece of sawed timber, differing from a board only
      in being thicker. See Board.
      [1913 Webster]

   2. Fig.: That which supports or upholds, as a board does a
      swimmer.
      [1913 Webster]

            His charity is a better plank than the faith of an
            intolerant and bitter-minded bigot.   --Southey.
      [1913 Webster]

   3. One of the separate articles in a declaration of the
      principles of a party or cause; as, a plank in the
      national platform. [Cant]
      [1913 Webster]

   Plank road, or Plank way, a road surface formed of
      planks. [U.S.]

   To walk the plank, to walk along a plank laid across the
      bulwark of a ship, until one overbalances it and falls
      into the sea; -- a method of disposing of captives
      practiced by pirates.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Plank \Plank\, v. t. [imp. & p. p. Planked; p. pr. & vb. n.
   Planking.]
   1. To cover or lay with planks; as, to plank a floor or a
      ship. "Planked with pine." --Dryden.
      [1913 Webster]

   2. To lay down, as on a plank or table; to stake or pay cash;
      as, to plank money in a wager. [Colloq. U.S.]
      [1913 Webster]

   3. To harden, as hat bodies, by felting.
      [1913 Webster]

   4. (Wooden Manuf.) To splice together the ends of slivers of
      wool, for subsequent drawing.
      [1913 Webster]

   Planked shad, shad split open, fastened to a plank, and
      roasted before a wood fire.
      [1913 Webster]

WordNet (r) 3.0 (2006):

plank
    n 1: a stout length of sawn timber; made in a wide variety of
         sizes and used for many purposes [syn: board, plank]
    2: an endorsed policy in the platform of a political party
    v 1: cover with planks; "The streets were planked" [syn:
         plank, plank over]
    2: set (something or oneself) down with or as if with a noise;
       "He planked the money on the table"; "He planked himself into
       the sofa" [syn: plank, flump, plonk, plop, plunk,
       plump down, plunk down, plump]
    3: cook and serve on a plank; "Planked vegetable"; "Planked
       shad"

Moby Thesaurus II by Grady Ward, 1.0:

150 Moby Thesaurus words for "plank":
   Mystik tape, Scotch tape, adhesive tape, band, bandage, batten,
   beam, belt, billet, board, boarding, brick, cast, cellophane tape,
   chuck, clap, clapboard, cloth tape, coat, coating, collop, cord,
   cordwood, covering, cut, deal, declaration of policy, disk,
   driftwood, face, fascia, feuille, fillet, film, firewood, flap,
   fling, foil, fold, friction tape, girdle, glass, glaze, hardwood,
   hurl, issue, keynote address, keynote speech, lamella, lamina,
   laminated glass, laminated wood, lap, lath, lathing, lathwork,
   leaf, ligula, ligule, list, log, lumber, masking tape, membrane,
   pane, panel, panelboard, paneling, panelwork, paper,
   party platform, patina, peel, pellicle, plait, planking,
   plastic tape, plate, platform, plating, plop, plump, plunk, ply,
   plyboard, plywood, pole, post, program, puncheon, rasher, revet,
   ribband, ribbon, safety glass, scum, shake, sheathe, sheathing,
   sheathing board, sheet, sheeting, shingle, shred, sideboard,
   siding, skin, slab, slap, slat, slate, slice, slip, softwood,
   spill, splat, spline, stave, stick, stick of wood, stone,
   stovewood, strake, strap, strip, strop, table, tablet, taenia,
   tape, tape measure, tapeline, thatch, three-by-four, throw, thrust,
   ticker tape, tile, timber, timbering, timberwork, toss,
   two-by-four, veneer, wafer, wall in, wall up, wallpaper,
   weatherboard, wood