The Collaborative International Dictionary of English v.0.48:

Guess \Guess\ (g[e^]s), v. t. [imp. & p. p. Guessed; p. pr. &
   vb. n. Guessing.] [OE. gessen; akin to Dan. gisse, Sw.
   gissa, Icel. gizha, D. gissen: cf. Dan. giette to guess,
   Icel. geta to get, to guess. Probably originally, to try to
   get, and akin to E. get. See Get.]
   1. To form an opinion concerning, without knowledge or means
      of knowledge; to judge of at random; to conjecture.
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            First, if thou canst, the harder reason guess.
                                                  --Pope.
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   2. To judge or form an opinion of, from reasons that seem
      preponderating, but are not decisive.
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            We may then guess how far it was from his design.
                                                  --Milton.
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            Of ambushed men, whom, by their arms and dress,
            To be Taxallan enemies I guess.       --Dryden.
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   3. To solve by a correct conjecture; to conjecture rightly;
      as, he who guesses the riddle shall have the ring; he has
      guessed my designs.
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   4. To hit upon or reproduce by memory. [Obs.]
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            Tell me their words, as near as thou canst guess
            them.                                 --Shak.
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   5. To think; to suppose; to believe; to imagine; -- followed
      by an objective clause.
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            Not all together; better far, I guess,
            That we do make our entrance several ways. --Shak.
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            But in known images of life I guess
            The labor greater.                    --Pope.

   Syn: To conjecture; suppose; surmise; suspect; divine; think;
        imagine; fancy.

   Usage: To Guess, Think, Reckon. Guess denotes, to
          attempt to hit upon at random; as, to guess at a thing
          when blindfolded; to conjecture or form an opinion on
          hidden or very slight grounds: as, to guess a riddle;
          to guess out the meaning of an obscure passage. The
          use of the word guess for think or believe, although
          abundantly sanctioned by good English authors, is now
          regarded as antiquated and objectionable by
          discriminating writers. It may properly be branded as
          a colloguialism and vulgarism when used respecting a
          purpose or a thing about which there is no
          uncertainty; as, I guess I 'll go to bed.
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WordNet (r) 3.0 (2006):

guessing
    n 1: an estimate based on little or no information [syn:
         guess, guesswork, guessing, shot, dead reckoning]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "guessing":
   abashed, abroad, adrift, astray, at sea, bewildered, bothered,
   clueless, confused, discomposed, disconcerted, dismayed,
   disoriented, distracted, distraught, disturbed, embarrassed,
   in a fix, in a maze, in a pickle, in a scrape, in a stew, lost,
   mazed, off the track, perturbed, put-out, turned around, upset,
   without a clue