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]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "inexplicable":
   aimless, baffling, bewildering, casual, causeless, confounding,
   designless, driftless, dysteleological, enigmatic,
   enwrapped in mystery, haphazard, incomprehensible, indecipherable,
   indefinable, indescribable, indiscriminate, inextricable,
   inscrutable, insoluble, insolvable, mindless, mysterious, mystic,
   mystifying, odd, peculiar, perplexing, promiscuous, purposeless,
   puzzling, random, stochastic, strange, stray, unaccountable,
   undefinable, undirected, unexplainable, unfathomable,
   unintelligible, uninterpretable, unmotivated, unsolvable