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®

illegibly
     adv : in an illegible manner; "this student writes illegibly"
           [syn: {undecipherably}, {unreadably}] [ant: {legibly}]