The Collaborative International Dictionary of English v.0.48:
Surmisable \Sur*mis"a*ble\ (s[^u]r*m[imac]z"[.a]*b'l), a. Capable of being surmised; as, a surmisable result. [1913 Webster]WordNet (r) 3.0 (2006):
surmisable adj 1: capable of being inferred on slight grounds [syn: presumable, supposable, surmisable]