Source : Webster's Revised Unabridged Dictionary (1913)

Supposable \Sup*pos"a*ble\, a.
   Capable of being supposed, or imagined to exist; as, that is
   not a supposable case. -- {Sup*pos"a*ble*ness}, n. --
   {Sup*pos"a*bly}, adv.

Source : WordNet®

supposable
     adj : able to be inferred on slight grounds [syn: {presumable}, {surmisable}]