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]