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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The Collaborative International Dictionary of English v.0.48:

Guess \Guess\, v. i.
   To make a guess or random judgment; to conjecture; -- with
   at, about, etc.
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         This is the place, as well as I may guess. --Milton.
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The Collaborative International Dictionary of English v.0.48:

Guess \Guess\, n.
   An opinion as to anything, formed without sufficient or
   decisive evidence or grounds; an attempt to hit upon the
   truth by a random judgment; a conjecture; a surmise.
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         A poet must confess
         His art 's like physic -- but a happy guess. --Dryden.
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WordNet (r) 3.0 (2006):

guess
    n 1: a message expressing an opinion based on incomplete
         evidence [syn: guess, conjecture, supposition,
         surmise, surmisal, speculation, hypothesis]
    2: an estimate based on little or no information [syn: guess,
       guesswork, guessing, shot, dead reckoning]
    v 1: expect, believe, or suppose; "I imagine she earned a lot of
         money with her new novel"; "I thought to find her in a bad
         state"; "he didn't think to find her in the kitchen"; "I
         guess she is angry at me for standing her up" [syn:
         think, opine, suppose, imagine, reckon, guess]
    2: put forward, of a guess, in spite of possible refutation; "I
       am guessing that the price of real estate will rise again";
       "I cannot pretend to say that you are wrong" [syn: guess,
       venture, pretend, hazard]
    3: judge tentatively or form an estimate of (quantities or
       time); "I estimate this chicken to weigh three pounds" [syn:
       estimate, gauge, approximate, guess, judge]
    4: guess correctly; solve by guessing; "He guessed the right
       number of beans in the jar and won the prize" [syn: guess,
       infer]

Moby Thesaurus II by Grady Ward, 1.0:

173 Moby Thesaurus words for "guess":
   account as, answer, appraise, appreciate, assess, assume,
   assumption, be afraid, believe, blind bargain, blind guess,
   bold conjecture, borderline case, bottom, call, cast a horoscope,
   cast a nativity, chance, class, clear up, conceive, conclude,
   conjecture, consider, contingency, crack, daresay, debug, decipher,
   decode, deduce, deem, disentangle, divine, do, dope, dope out,
   double contingency, dowse for water, esteem, estimate, evaluate,
   expect, explain, fancy, fathom, feel, feeling, figure, figure out,
   find out, find the answer, find the solution, forebode, forecast,
   foresee, foretell, form an estimate, fortune-tell, gamble, gauge,
   get, get right, give a guess, give an appreciation, guess right,
   guesstimate, guesswork, hariolate, have a hunch, have an idea,
   have an impression, have an inkling, have it, have the idea,
   hazard a conjecture, hit it, hold, hold as, hunch, hypothesis,
   hypothesize, imagine, interpret, judge, judgement, look upon as,
   maintain, make a prediction, make a prognosis, make a prophecy,
   make an estimation, make out, mark, measure, open question,
   open the lock, opine, perhaps, piece of guesswork, plumb,
   postulate, predict, prefigure, presage, presume, pretend, prize,
   prognosticate, prophesy, psych, psych out, puzzle out, question,
   rank, rate, ravel, ravel out, read palms, read tea leaves,
   read the future, reason, reckon, regard, resolve, riddle,
   risk assuming, rough guess, set down as, shot,
   sight-unseen transaction, solve, soothsay, sort out, speculate,
   speculation, stab, suppose, supposition, surmise, suspect,
   suspicion, take, take for, take it, tell fortunes, tell the future,
   tentatively suggest, theory, think, toss-up, touch and go, trow,
   undecided issue, undo, unlock, unravel, unriddle, unscramble,
   untangle, untwist, unverified supposition, unweave, valuate, value,
   vaticinate, venture a guess, view as, wager, ween, wild guess,
   work, work out