adjective.
Difficult to understand or interpret; impenetrable.
The Century Dictionary and Cyclopedia
Incapable of being searched into or scrutinized; impenetrable to inquiry or investigation; incognizable: as, the ways of Providence are often inscrutable.
the GNU version of the Collaborative International Dictionary of English
adjective.
Unsearchable; incapable of being searched into and understood by inquiry or study; impossible or difficult to be explained or accounted for satisfactorily; obscure; incomprehensible.