The Collaborative International Dictionary of English v.0.48:

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.
   [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]

WordNet (r) 3.0 (2006):

inscrutable
    adj 1: of an obscure nature; "the new insurance policy is
           written without cryptic or mysterious terms"; "a deep
           dark secret"; "the inscrutable 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, inscrutable, mysterious,
           mystifying]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "inscrutable":
   Greek to one, ambiguous, arcane, beyond understanding, cabalistic,
   impenetrable, inarticulate, incognizable, incoherent,
   incomprehensible, mystic, numinous, past comprehension, rambling,
   unaccountable, unconnected, unfathomable, unguessed,
   unintelligible, unknowable, unsearchable