inexertion comes from in- ('not') + exertion, from exert, from Latin exsertus, past participle of exserere ('to thrust out'), from ex- ('out') + serere ('to join, put together'). 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 noun sense is "lack of exertion or effort", so the historical development ends in a clearly identifiable modern dictionary use. The same base also appears in related forms such as exertion and exert. Its semantic neighborhood includes inactivity, idleness, indolence, and sloth, which helps locate the modern sense without adding a separate etymological claim. The named historical forms include Latin exserere and Latin serere.