Illiterate means unable to read or write. Not slow at it — unable.
The word describes a circumstance rather than an ability, since most illiterate adults never had schooling rather than failing at it, and that distinction matters because the word is often thrown as an insult. Used loosely it means badly written, as in an illiterate email. English has extended the pattern to other kinds of ignorance, giving us computer illiterate and financially illiterate, and those uses are much gentler. The noun is illiteracy, and the opposite is literate.
1adjectiveilliterateUnable to read or write.
“Nearly half the village was illiterate.”
2adjectiveignorantIgnorant of a particular subject.
“He cheerfully admits he is computer illiterate.”
Illiterate is unlettered. Latin litterātus meant furnished with letters, from littera, and in Middle English illiterat meant ignorant of Latin in particular rather than unable to read at all.
Latin
illīterātus
Middle English
illiterat — uneducated, ignorant of Latin
English, today
illiterate
Built from
r, in Middle English; from Middle English illiterat (“uneducated, ignorant of Latin”), borrowed from Latin illīterātus, illitterātus (“unlearned, ignorant”), itself from in- (“un-”) + līterātus, litterātus (“furnished with letters”) (see -ate), from lītera, littera (“letter, character”). The noun was derived from the adjective by substantivization, see -ate.
Etymology from Wiktionary, CC BY-SA