The Collaborative International Dictionary of English v.0.48:

Presumptuous \Pre*sump"tu*ous\ (?; 135), a. [L. praesumptuosus:
   cf. F. pr['e]somptueux, OF. also presumptuous. See
   Presumption.]
   1. Full of presumption; presuming; overconfident or
      venturesome; audacious; rash; taking liberties unduly;
      arrogant; insolent; as, a presumptuous commander;
      presumptuous conduct.
      [1913 Webster]

            A class of presumptuous men, whom age has not made
            cautious, nor adversity wise.         --Buckminster.
      [1913 Webster]

   2. Founded on presumption; as, a presumptuous idea. "False,
      presumptuous hope." --Milton.
      [1913 Webster]

   3. Done with hold design, rash confidence, or in violation of
      known duty; willful. "Keep back the servant also from
      presumptuous sins." --Ps. xix. 13.
      [1913 Webster]

   Syn: Overconfident; foolhardy; rash; presuming; forward;
        arrogant; insolent.
        [1913 Webster]

WordNet (r) 3.0 (2006):

presumptuous
    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]

Moby Thesaurus II by Grady Ward, 1.0:

77 Moby Thesaurus words for "presumptuous":
   acceptable, adventurous, arrogant, assuming, audacious, bold,
   brash, brazen, bumptious, busy, busybody, cheeky, cold, complacent,
   confident, contumelious, cool, daredevil, daring, death-defying,
   disdainful, egotistical, familiar, fire-eating, foolhardy, forward,
   gay, harebrained, hubristic, immodest, impertinent, impudent,
   inexcusable, inquisitive, insolent, insulting, licentious, lofty,
   madbrain, madbrained, madcap, meddlesome, meddling, nosy,
   obtrusive, officious, outrageous, overconfident, overpresumptuous,
   overweening, pompous, presuming, pretentious, prideful, procacious,
   proud, prying, pushful, pushing, pushy, saucy, self-appointed,
   self-asserting, self-assertive, self-assured, self-conceited,
   self-elect, self-satisfied, smug, snoopy, supercilious,
   temerarious, uppish, uppity, wild, wild-ass, would-be