Wiktionary
unsurmisable
a. cannot be surmised, unable to be surmised.
Usage examples of "unsurmisable".
Moment by moment, the coincidence on which I had stumbled appeared all too bizarre for belief, and I was troubled now by the monstrous and absurd idea that the Mariel I knew had only just died, and that this fantastic city was in some unsurmisable manner connected with her death.