The Collaborative International Dictionary of English v.0.48:

Suppose \Sup*pose"\, v. t. [imp. & p. p. Supposed; p. pr. &
   vb. n. Supposing.] [F. supposer; pref. sub- under + poser
   to place; -- corresponding in meaning to L. supponere,
   suppositum, to put under, to substitute, falsify,
   counterfeit. See Pose.]
   1. To represent to one's self, or state to another, not as
      true or real, but as if so, and with a view to some
      consequence or application which the reality would involve
      or admit of; to imagine or admit to exist, for the sake of
      argument or illustration; to assume to be true; as, let us
      suppose the earth to be the center of the system, what
      would be the result?
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            Suppose they take offence without a cause. --Shak.
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            When we have as great assurance that a thing is, as
            we could possibly, supposing it were, we ought not
            to make any doubt of its existence.   --Tillotson.
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   2. To imagine; to believe; to receive as true.
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            How easy is a bush supposed a bear!   --Shak.
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            Let not my lord suppose that they have slain all the
            young men, the king's sons; for Amnon only is dead.
                                                  --2 Sam. xiii.
                                                  32.
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   3. To require to exist or to be true; to imply by the laws of
      thought or of nature; as, purpose supposes foresight.
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            One falsehood always supposes another, and renders
            all you can say suspected.            --Female
                                                  Quixote.
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   4. To put by fraud in the place of another. [Obs.]
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   Syn: To imagine; believe; conclude; judge; consider; view;
        regard; conjecture; assume.
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WordNet (r) 3.0 (2006):

supposed
    adj 1: required or under orders; "I'm supposed to be there at
           ten"; "he was supposed to go to the store"
    2: mistakenly believed; "the supposed existence of ghosts"
    3: doubtful or suspect; "these so-called experts are no help"
       [syn: alleged(a), so-called, supposed]
    4: based primarily on surmise rather than adequate evidence;
       "theories about the extinction of dinosaurs are still highly
       conjectural"; "the supposed reason for his absence";
       "suppositious reconstructions of dead languages";
       "hypothetical situation" [syn: conjectural, divinatory,
       hypothetical, hypothetic, supposed, suppositional,
       suppositious, supposititious]

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "supposed":
   academic, accounted as, alleged, assumed, assumptive, conjectural,
   conjectured, deemed, expected, given, granted, hinted,
   hypothetical, imagined, implicated, implied, indicated, inferred,
   intended, intimated, involved, meant, obliged, ostensible,
   postulated, postulational, postulatory, premised, presumed,
   presumptive, presupposed, pretended, professed, provisional,
   purported, putative, reputed, required, so-called, speculative,
   suggested, suppositional, supposititious, suppositive, suppository,
   taken for granted, tentative, theoretical, understood