The Collaborative International Dictionary of English v.0.48:

Presumable \Pre*sum"a*ble\, a. [Cf. F. pr['e]sumable.]
   Such as may be presumed or supposed to be true; that seems
   entitled to belief without direct evidence.
   [1913 Webster]

WordNet (r) 3.0 (2006):

presumable
    adj 1: capable of being inferred on slight grounds [syn:
           presumable, supposable, surmisable]