adj. Difficult or impossible to comprehend, fathom, or interpret.
n. One who or that which is inscrutable; a person, etc. that cannot be comprehended.
the GNU version of the Collaborative International Dictionary of English
adj. Unsearchable; incapable of being searched into and understood by inquiry or study; impossible or difficult to be explained or accounted for satisfactorily; obscure; incomprehensible.
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.