The Collaborative International Dictionary of English v.0.48:

Inexplicable \In*ex"pli*ca*ble\, a. [L. inexplicabilis: cf. F.
   inexplicable. See In- not, and Explicable.]
   Not explicable; not explainable; incapable of being
   explained, interpreted, or accounted for; as, an inexplicable
   mystery. "An inexplicable scratching." --Cowper.
   [1913 Webster]

         Their reason is disturbed; their views become vast and
         perplexed, to others inexplicable, to themselves
         uncertain.                               --Burke.
   [1913 Webster]

WordNet (r) 3.0 (2006):

inexplicable
    adj 1: incapable of being explained or accounted for;
           "inexplicable errors"; "left the house at three in the
           morning for inexplicable reasons" [syn: inexplicable,
           incomprehensible] [ant: explicable]