Etymology
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illegible (adj.)
1630s, from assimilated form of in- (1) "not, opposite of" + legible. Related: Illegibly; illegibility.
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unreadable (adj.)

1787, of written material, "dull, distasteful," from un- (1) "not" + readable (adj.). Meaning "illegible" is from 1830, but is better left to illegible.

The illegible is not plain enough to be deciphered; the unreadable is not interesting enough to be perused. [Fowler]
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