The Collaborative International Dictionary of English v.0.48:

Unknowable \Unknowable\
   See knowable.

WordNet (r) 3.0 (2006):

unknowable
    adj 1: not knowable; "the unknowable mysteries of life" [ant:
           cognisable, cognizable, cognoscible, knowable]

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "unknowable":
   Greek to one, able, ambiguous, arcane, beyond understanding,
   cabalistic, enigmatic, impenetrable, inarticulate, incalculable,
   incognizable, incoherent, incomprehensible, inscrutable,
   mysterious, mystic, numinous, past comprehension, puzzling,
   rambling, sealed, strange, unapparent, unapprehended,
   unascertained, unbeknown, uncharted, unclassified, unconnected,
   undisclosed, undiscoverable, undiscovered, undivulged, unexplained,
   unexplored, unexposed, unfamiliar, unfathomable, unfathomed,
   ungraspable, unguessed, unheard, unheard-of, unidentified,
   unimaginable, unintelligible, uninvestigated, unknown, unperceived,
   unplumbed, unrevealed, unsearchable, unsuspected, untouched,
   virgin