The Collaborative International Dictionary of English v.0.48:

Indecipherable \In`de*ci"pher*a*ble\, a.
   Not decipherable; incapable of being deciphered, explained,
   or solved. -- In`de*ci"pher*a*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

indecipherable
    adj 1: not easily deciphered; "indecipherable handwriting" [syn:
           indecipherable, unclear, undecipherable,
           unreadable]
    2: impossible to determine the meaning of; "an indecipherable
       message"