Source : Webster's Revised Unabridged Dictionary (1913)

Illegible \Il*leg"i*ble\, a.
   Incapable of being read; not legible; as, illegible
   handwriting; an illegible inscription. --
   {Il*leg"i*ble*ness}, n. -- {Il*leg"i*bly}, adv.

Source : WordNet®

illegible
     adj : (of handwriting, print, etc.) not legible; "illegible
           handwriting" [ant: {legible}]