means A state of severe mental disturbance or irrationality; by extension, wild folly or chaotic behavior.

from From "mad," which traces back to Old English "gemǣded," meaning "made insane" — a past participle of a verb related to "gemād," "foolish, mad." These connect to a Proto-Germanic root "maidjaną," "to change, spoil, injure," with possible cousins meaning "crippled" or "changed for the worse." So the oldest sense isn't fury but alteration — a mind shifted off its expected shape. The "-ness" suffix simply turns the adjective into a state, the way "sad" becomes "sadness."