The Collaborative International Dictionary of English v.0.48:

Forestall \Fore*stall"\, v. t. [imp. & p. p. Forestalled; p.
   pr. & vb. n. Forestalling.] [OE. forstallen to stop, to
   obstruct; to stop (goods) on the way to the market by buying
   them beforehand, from forstal obstruction, AS. forsteal,
   foresteall, prop., a placing one's self before another. See
   Fore, and Stall.]
   1. To take beforehand, or in advance; to anticipate.
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            What need a man forestall his date of grief,
            And run to meet what he would most avoid? --Milton.
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   2. To take possession of, in advance of some one or something
      else, to the exclusion or detriment of the latter; to get
      ahead of; to preoccupy; also, to exclude, hinder, or
      prevent, by prior occupation, or by measures taken in
      advance.
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            An ugly serpent which forestalled their way.
                                                  --Fairfax.
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            But evermore those damsels did forestall
            Their furious encounter.              --Spenser.
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            To be forestalled ere we come to fall. --Shak.
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            Habit is a forestalled and obstinate judge. --Rush.
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   3. To deprive; -- with of. [R.]
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            All the better; may
            This night forestall him of the coming day! --Shak.
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   4. (Eng. Law) To obstruct or stop up, as a way; to stop the
      passage of on highway; to intercept on the road, as goods
      on the way to market.
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   To forestall the market, to buy or contract for merchandise
      or provision on its way to market, with the intention of
      selling it again at a higher price; to dissuade persons
      from bringing their goods or provisions there; or to
      persuade them to enhance the price when there. This was an
      offense at law in England until 1844. --Burrill.

   Syn: To anticipate; monopolize; engross.
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WordNet (r) 3.0 (2006):

forestall
    v 1: keep from happening or arising; make impossible; "My sense
         of tact forbids an honest answer"; "Your role in the
         projects precludes your involvement in the competitive
         project" [syn: prevent, forestall, foreclose,
         preclude, forbid]
    2: act in advance of; deal with ahead of time [syn:
       anticipate, foresee, forestall, counter]

Moby Thesaurus II by Grady Ward, 1.0:

95 Moby Thesaurus words for "forestall":
   anticipate, avert, bamboozle, bar, beguile, betray, bluff, cajole,
   calculate on, cheat on, circumvent, conjure, corner, count on,
   debar, deceive, deflect, delay, delude, deter, diddle, discourage,
   dishearten, double-cross, dupe, engross, estop, exclude, fend,
   fend off, forbid, foreclose, foreglimpse, forerun, foresee,
   foretaste, frustrate, gammon, get ahead of, get around, go before,
   go off half-cocked, gull, help, hinder, hoax, hocus-pocus, hog,
   hornswaggle, humbug, intercept, juggle, jump the gun, keep from,
   keep off, let down, look for, look forward to, look out for,
   look to, mock, monopolize, obstruct, obviate, outmaneuver,
   outreach, outsmart, outwit, overreach, parry, pigeon,
   play one false, preclude, prevent, prohibit, put something over,
   reckon on, repel, rule out, save, snow, stave off, stop,
   string along, take in, take it all, thwart, tie up, trick,
   turn aside, two-time, ward, ward off, watch out for,
   win the start