hardness comes from Old English heardnes, from heard ('hard') plus the noun-forming suffix -ness, from Proto-Germanic *harduz. 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 quality of being firm, solid, or unyielding", so the historical development ends in a clearly identifiable modern dictionary use. The same base also appears in related forms such as hard, harden, and hardy. Its semantic neighborhood includes firmness, solidity, rigidity, and toughness, which helps locate the modern sense without adding a separate etymological claim. The named historical forms include Old English heard and Proto-Germanic *harduz.