Meaning

noun the property of being rigid and resistant to pressure; not easily scratched; measured on Mohs scale

noun a quality of water that contains dissolved mineral salts that prevent soap from lathering

noun devoid of passion or feeling; hardheartedness

originFrom Middle English hardness, hardnesse, from Old English heardnes, from heard + -nes, equivalent to hard + -ness.

Etymology from Wiktionary, CC BY-SA 3.0