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"