English Etymology From illegible + -ly. Pronunciation This entry needs pronunciation information. If you are familiar with the IPA or enPR then please add some! Audio (Southern England):(file) Adverb illegibly (comparative more illegibly, superlative most illegibly) In a manner that produces illegible results. The doctor illegibly scrawled his signature: if it weren't on his letterhead, no-one would know who had written it.Wikiwand - on Seamless Wikipedia browsing. On steroids.