The Collaborative International Dictionary of English v.0.48:

Presumptive \Pre*sump"tive\, a. [Cf. F. pr['e]somptif.]
   1. Based on presumption or probability; grounded on probable
      evidence; probable; as, presumptive proof.
      [1913 Webster]

   2. Presumptuous; arrogant. [R.] --Sir T. Browne.
      [1913 Webster]

   Presumptive evidence (Law), that which is derived from
      circumstances which necessarily or usually attend a fact,
      as distinct from direct evidence or positive proof;
      indirect or circumstantial evidence. "Presumptive evidence
      of felony should be cautiously admitted." --Blackstone.
      The distinction, however, between direct and presumptive
      (or circumstantial) evidence is now generally abandoned;
      all evidence being now more or less direct and more or
      less presumptive.

   Presumptive heir. See Heir presumptive, under Heir.
      [1913 Webster]

WordNet (r) 3.0 (2006):

presumptive
    adj 1: having a reasonable basis for belief or acceptance; "the
           presumptive heir (or heir apparent)"
    2: affording reasonable grounds for belief or acceptance;
       "presumptive evidence"; "a strong presumptive case is made
       out"

Moby Thesaurus II by Grady Ward, 1.0:

94 Moby Thesaurus words for "presumptive":
   absolute, acceptable, accounted as, adducible, admissible, alleged,
   apt, assumed, assumptive, attestative, attestive, authentic,
   based on, believable, certain, circumstantial, conceivable,
   conclusive, conjectured, convincing, credible, cumulative, damning,
   decisive, deemed, determinative, documentary, documented,
   evidential, evidentiary, ex parte, eye-witness, factual, fair,
   final, firsthand, foreseeable, founded on, given, granted,
   grounded on, hearsay, hopeful, implicit, in the cards,
   incontrovertible, indicative, indisputable, inferred, irrefutable,
   irresistible, justifiable, liable, likely, material, nuncupative,
   odds-on, overwhelming, plausible, postulated, postulational,
   predicated, predictable, predictable within limits, predicted,
   premised, presumable, presumed, probable, probative, promising,
   putative, rational, reasonable, reliable, reputed, sensible,
   significant, sound, statistically probable, suggestive, supposed,
   suppositional, supposititious, suppositive, sure, symptomatic,
   taken for granted, telling, tenable, understood, valid,
   verisimilar, weighty