Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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