The Collaborative International Dictionary of English v.0.48:

Assume \As*sume"\, v. t. [imp. & p. p. Assumed; p. pr. & vb.
   n. Assuming.] [L. assumere; ad + sumere to take; sub +
   emere to take, buy: cf. F. assumer. See Redeem.]
   1. To take to or upon one's self; to take formally and
      demonstratively; sometimes, to appropriate or take
      unjustly.
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            Trembling they stand while Jove assumes the throne.
                                                  --Pope.
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            The god assumed his native form again. --Pope.
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   2. To take for granted, or without proof; to suppose as a
      fact; to suppose or take arbitrarily or tentatively.
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            The consequences of assumed principles. --Whewell.
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   3. To pretend to possess; to take in appearance.
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            Ambition assuming the mask of religion. --Porteus.
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            Assume a virtue, if you have it not.  --Shak.
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   4. To receive or adopt.
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            The sixth was a young knight of lesser renown and
            lower rank, assumed into that honorable company.
                                                  --Sir W.
                                                  Scott.
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   Syn: To arrogate; usurp; appropriate.
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The Collaborative International Dictionary of English v.0.48:

Assuming \As*sum"ing\, a.
   Pretentious; taking much upon one's self; presumptuous.
   --Burke.
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WordNet (r) 3.0 (2006):

assuming
    adj 1: excessively forward; "an assumptive person"; "on a
           subject like this it would be too assuming for me to
           decide"; "the duchess would not put up with presumptuous
           servants" [syn: assumptive, assuming, presumptuous]