Source : Webster's Revised Unabridged Dictionary (1913)

Inscrutable \In*scru"ta*ble\, 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.

         '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.

Source : WordNet®

inscrutable
     adj : of an obscure nature; "the new insurance policy is written
           without cryptic or mysterious terms"; "a deep dark
           secret"; "the inscrutible workings of Providence"; "in
           its mysterious past it encompasses all the dim origins
           of life"- Rachel Carson; "rituals totally mystifying to
           visitors from other lands" [syn: {cryptic}, {cryptical},
            {deep}, {mysterious}, {mystifying}]