The Collaborative International Dictionary of English v.0.48:

Surmise \Sur*mise"\, v. t. [imp. & p. p. Surmised; p. pr. &
   vb. n. Surmising.]
   To imagine without certain knowledge; to infer on slight
   grounds; to suppose, conjecture, or suspect; to guess.
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         It wafted nearer yet, and then she knew
         That what before she but surmised, was true. --Dryden.
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         This change was not wrought by altering the form or
         position of the earth, as was surmised by a very
         learned man, but by dissolving it.       --Woodward.
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The Collaborative International Dictionary of English v.0.48:

Surmise \Sur*mise"\ (s[^u]r*m[imac]z"), n. [OF. surmise
   accusation, fr. surmettre, p. p. surmis, to impose, accuse;
   sur (see Sur-) + mettre to put, set, L. mittere to send.
   See Mission.]
   1. A thought, imagination, or conjecture, which is based upon
      feeble or scanty evidence; suspicion; guess; as, the
      surmises of jealousy or of envy.
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            [We] double honor gain
            From his surmise proved false.        --Milton.
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            No man ought to be charged with principles he
            actually disowns, unless his practicies contradict
            his profession; not upon small surmises. --Swift.
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   2. Reflection; thought. [Obs.] --Shak.
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   Syn: Conjecture; supposition; suspicion; doubt.
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WordNet (r) 3.0 (2006):

surmise
    n 1: a message expressing an opinion based on incomplete
         evidence [syn: guess, conjecture, supposition,
         surmise, surmisal, speculation, hypothesis]
    v 1: infer from incomplete evidence
    2: imagine to be the case or true or probable; "I suspect he is
       a fugitive"; "I surmised that the butler did it" [syn:
       suspect, surmise]

Moby Thesaurus II by Grady Ward, 1.0:

100 Moby Thesaurus words for "surmise":
   account as, assume, assumption, axiom, be afraid, believe,
   blind guess, bold conjecture, conceive, conclude, conclusion,
   conjecture, consider, daresay, deduce, deem, divine, dream, esteem,
   estimate, expect, fancy, feel, feeling, gather, grant, guess,
   guesswork, have a hunch, have an idea, have an impression,
   have an inkling, have the idea, hold, hold as, hunch, hypothesis,
   hypothesize, imagine, infer, inference, judge, let, let be,
   look upon as, maintain, notion, opine, perhaps, postulate,
   postulation, postulatum, prefigure, premise, presume, presumption,
   presupposal, presuppose, presupposition, presurmise, pretend,
   proposition, provisionally accept, reckon, regard, repute,
   risk assuming, rough guess, say, sense, set down as,
   set of postulates, shot, speculate, speculation, stab, supposal,
   suppose, supposing, supposition, suspect, suspicion, take,
   take for, take for granted, take it, take to be, theorize, theory,
   thesis, think, trow, understand, understanding,
   unverified supposition, venture a guess, view as, ween, wild guess,
   working hypothesis