inmost comes from Old English innemest ('innermost'), a double superlative formed from inne ('within') with the superlative suffixes -m- and -est; the modern spelling is shaped by association with 'most'. 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 adjective sense is "farthest within; most intimate or private", so the historical development ends in a clearly identifiable modern dictionary use. The same base also appears in related forms such as innermost and in. Its semantic neighborhood includes innermost, deepest, intimate, and secret, which helps locate the modern sense without adding a separate etymological claim. The named historical forms include Old English innemest and Old English inne.