inscrutable

a. [ L. inscrutabilis : cf. F. inscrutable. See In- not, and Scrutiny. ] Unsearchable; incapable of being searched into and understood by inquiry or study; impossible or difficult to be explained or accounted for satisfactorily; obscure; incomprehensible; as, an inscrutable design or event. [ 1913 Webster ]

'T is not in man
To yield a reason for the will of Heaven
Which is inscrutable. Beau. & Fl. [ 1913 Webster ]

Waiving a question so inscrutable as this. De Quincey. [ 1913 Webster ]