insaneness comes from insane (from Latin insanus 'unsound, mad', from in- 'not' + sanus 'healthy, sound') + the noun suffix -ness. Morphologically, the ending is part of the etymology: the suffix is attached to the older base named in the history, producing a later English derivative. In present English, the noun sense is "the state or quality of being insane; madness", so the historical development ends in a clearly identifiable modern dictionary use. The same base also appears in related forms such as insane, sane, sanity, and insanity. Its semantic neighborhood includes insanity, madness, lunacy, and derangement, which helps locate the modern sense without adding a separate etymological claim. The named historical forms include Latin in- and Latin sanus.