The Collaborative International Dictionary of English v.0.48:

Conjecture \Con*jec"ture\, v. t. [imp. & p. p. Conjectured; p.
   pr. & vb. n. Conjecturing.] [Cf. F. conjecturer. Cf.
   Conject.]
   To arrive at by conjecture; to infer on slight evidence; to
   surmise; to guess; to form, at random, opinions concerning.
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         Human reason can then, at the best, but conjecture what
         will be.                                 --South.
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