The Collaborative International Dictionary of English v.0.48:

Prop \Prop\, n. [Akin to LG., D., & Dan. prop stopple, stopper,
   cork, Sw. propp, G. pfropf. See Prop, v.]
   That which sustains an incumbent weight; that on which
   anything rests or leans for support; a support; a stay; as, a
   prop for a building. "Two props of virtue." --Shak.
   [1913 Webster] Propaedeutic

The Collaborative International Dictionary of English v.0.48:

Prop \Prop\, n.
   A shell, used as a die. See Props.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Prop \Prop\, v. t. [imp. & p. p. Propped; p. pr. & vb. n.
   Propping.] [Akin to LG. & D. proppen to cram, stuff, thrust
   into, stop, G. pfropfen, Dan. proppe, Sw. proppa; of
   uncertain origin, cf. G. pfropfen to graft, fr. L. propago
   set, layer of a plant, slip, shoot. Cf. 3d. Prop,
   Propagate.]
   To support, or prevent from falling, by placing something
   under or against; as, to prop up a fence or an old building;
   (Fig.) to sustain; to maintain; as, to prop a declining
   state. --Shak.
   [1913 Webster]

         Till the bright mountains prop the incumbent sky.
                                                  --Pope.
   [1913 Webster]

         For being not propp'd by ancestry.       --Shak.
   [1913 Webster]

         I prop myself upon those few supports that are left me.
                                                  --Pope.
   [1913 Webster]

WordNet (r) 3.0 (2006):

prop
    n 1: a support placed beneath or against something to keep it
         from shaking or falling
    2: any movable articles or objects used on the set of a play or
       movie; "before every scene he ran down his checklist of
       props" [syn: property, prop]
    3: a propeller that rotates to push against air [syn: airplane
       propeller, airscrew, prop]
    v 1: support by placing against something solid or rigid; "shore
         and buttress an old building" [syn: prop up, prop,
         shore up, shore]

Moby Thesaurus II by Grady Ward, 1.0:

138 Moby Thesaurus words for "prop":
   advocate, afford support, airscrew, alpenstock, arm,
   athletic supporter, back, back up, backbone, backing, bandeau,
   bear, bear out, bear up, bearer, beef up, blackface, bolster,
   bolster up, bra, brace, brace up, bracer, bracket, brassiere,
   buoy up, buttress, cane, carrier, carry, case harden, cervix,
   clown white, column, confirm, corset, costume, cradle, crook,
   crutch, cushion, driver, fan, finance, fortify, foundation garment,
   fulcrum, fund, gird, girdle, give support, greasepaint, guy,
   guywire, harden, hold, hold up, impeller, invigorate, jock,
   jockstrap, keep, keep afloat, keep up, lean, lend support,
   mainstay, maintain, maintainer, makeup, mast, neck, nerve,
   paddle wheel, pillow, piston, practical piece, prop up, propellant,
   propeller, property, propulsor, refresh, reinforce, reinforcement,
   reinforcer, reinvigorate, rest, resting place, restrengthen,
   rigging, rotor, screw, screw propeller, shore, shore up, shoulder,
   shroud, spine, sprit, staff, stand, standing rigging, stave, stay,
   steel, stick, stiffen, stiffener, strengthen, strengthener,
   subsidize, subvention, subventionize, support, supporter, sustain,
   sustainer, temper, theatrical makeup, toughen, truss, turbine,
   twin screws, underbrace, undergird, underlie, underpin,
   underpinning, underset, upbear, uphold, upholder, upkeep, upright,
   vertical, walking stick, wheel