The Collaborative International Dictionary of English v.0.48:

Surmisal \Sur*mis"al\ (s[^u]r*m[imac]z"al), n.
   Surmise. [R.] --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

surmisal
    n 1: a message expressing an opinion based on incomplete
         evidence [syn: guess, conjecture, supposition,
         surmise, surmisal, speculation, hypothesis]