The Collaborative International Dictionary of English v.0.48:
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. [1913 Webster]WordNet (r) 3.0 (2006):
illegibly adv 1: in an illegible manner; "this student writes illegibly" [syn: illegibly, undecipherably, unreadably] [ant: decipherably, legibly, readably]