illiterate comes from Latin illitteratus ('unlearned, ignorant of letters'), from in- ('not') + litteratus ('learned'), from littera ('letter'). Morphologically, the prefixed shape matters: the initial element belongs to the recorded formation and helps explain the modern form's surface analysis. In present English, the adjective sense is "unable to read or write; uneducated", so the historical development ends in a clearly identifiable modern dictionary use. The same base also appears in related forms such as literate, literacy, and letter, and in derivatives such as illiterately and illiteracy. Its semantic neighborhood includes unlettered, uneducated, ignorant, and unschooled, which helps locate the modern sense without adding a separate etymological claim. The named historical forms include Latin illitteratus and Latin littera.