Inscrutable, a.
Etymology: [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. 'T is not in man To yield a reason for the will of Heaven Which is inscrutable. Beau. & Fl. Waiving a question so inscrutable as this. De Quincey.
Shades of meaning
Fernald has no entry of its own for inscrutable; he lists it under mysterious.
He lists it without comment; the entries above explain its neighbors.