The Collaborative International Dictionary of English v.0.48:

Bolster \Bol"ster\ (b[=o]l"st[~e]r; 110), n. [AS. bolster; akin
   to Icel. b[=o]lstr, Sw. & Dan. bolster, OHG. bolstar,
   polstar, G. polster; from the same root as E. bole stem, bowl
   hollow vessel. Cf. Bulge, Poltroon.]
   1. A long pillow or cushion, used to support the head of a
      person lying on a bed; -- generally laid under the
      pillows.
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            And here I'll fling the pillow, there the bolster,
            This way the coverlet, another way the sheets.
                                                  --Shak.
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   2. A pad, quilt, or anything used to hinder pressure, support
      any part of the body, or make a bandage sit easy upon a
      wounded part; a compress.
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            This arm shall be a bolster for thy head. --Gay.
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   3. Anything arranged to act as a support, as in various forms
      of mechanism, etc.
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   4. (Saddlery) A cushioned or a piece part of a saddle.
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   5. (Naut.)
      (a) A cushioned or a piece of soft wood covered with
          tarred canvas, placed on the trestletrees and against
          the mast, for the collars of the shrouds to rest on,
          to prevent chafing.
      (b) Anything used to prevent chafing.
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   6. A plate of iron or a mass of wood under the end of a
      bridge girder, to keep the girder from resting directly on
      the abutment.
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   7. A transverse bar above the axle of a wagon, on which the
      bed or body rests.
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   8. The crossbeam forming the bearing piece of the body of a
      railway car; the central and principal cross beam of a car
      truck.
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   9. (Mech.) the perforated plate in a punching machine on
      which anything rests when being punched.
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   10. (Cutlery)
       (a) That part of a knife blade which abuts upon the end
           of the handle.
       (b) The metallic end of a pocketknife handle. --G.
           Francis.
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   11. (Arch.) The rolls forming the ends or sides of the Ionic
       capital. --G. Francis.
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   12. (Mil.) A block of wood on the carriage of a siege gun,
       upon which the breech of the gun rests when arranged for
       transportation.

   Note: [See Illust. of Gun carriage.]
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   Bolster work (Arch.), members which are bellied or curved
      outward like cushions, as in friezes of certain classical
      styles.
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The Collaborative International Dictionary of English v.0.48:

Bolster \Bol"ster\, v. t. [imp. & p. p. Bolstered; p. pr. &
   vb. n. Bolstering.]
   1. To support with a bolster or pillow. --S. Sharp.
      [1913 Webster]

   2. To support, hold up, or maintain with difficulty or
      unusual effort; -- often with up.
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            To bolster baseness.                  --Drayton.
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            Shoddy inventions designed to bolster up a
            factitious pride.                     --Compton
                                                  Reade.
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WordNet (r) 3.0 (2006):

bolster
    n 1: a pillow that is often put across a bed underneath the
         regular pillows [syn: bolster, long pillow]
    v 1: support and strengthen; "bolster morale" [syn: bolster,
         bolster up]
    2: prop up with a pillow or bolster
    3: add padding to; "pad the seat of the chair" [syn: pad,
       bolster]

Moby Thesaurus II by Grady Ward, 1.0:

92 Moby Thesaurus words for "bolster":
   advance, affirm, afford support, aid, air mattress, assist, assure,
   attest, authenticate, back, back up, bear, bear out, bear up,
   bedding, bolster up, brace, brace up, buck up, buoy up, buttress,
   carry, certify, cheer, circumstantiate, confirm, corroborate,
   cradle, crutch, cushion, document, embolden, encourage, finance,
   fortify, fund, further, give support, hearten, help, hold, hold up,
   innerspring mattress, inspire, inspirit, keep, keep afloat,
   keep up, lend support, litter, mainstay, maintain, mat, mattress,
   nerve, pad, pallet, pillow, probate, prop, prop up, prove, ratify,
   reassure, reinforce, rug, shore, shore up, shoulder, sleeping bag,
   springs, stay, strengthen, subsidize, substantiate, subvention,
   subventionize, support, sustain, underbed, underbedding,
   underbrace, undergird, underlie, underpin, underset, upbear,
   uphold, upkeep, validate, verify, warrant



Easton's 1897 Bible Dictionary:

Bolster
   The Hebrew word _kebir_, rendered "pillow" in 1 Sam. 19:13, 16,
   but in Revised Version marg. "quilt" or "network," probably
   means some counterpane or veil intended to protect the head of
   the sleeper. A different Hebrew word (meraashoth') is used for
   "bolster" (1 Sam. 26:7, 11, 16). It is rightly rendered in
   Revised Version "at his head." In Gen. 28:11, 18 the Authorized
   Version renders it "for his pillows," and the Revised Version
   "under his head." In Ezek. 13:18, 20 another Hebrew word
   (kesathoth) is used, properly denoting "cushions" or "pillows,"
   as so rendered both in the Authorized and the Revised Version.