means the quality of being gentle, mild, or moderate — not harsh, severe, or intense, whether in temper, weather, or flavor.

from From Old English 'milde,' meaning gentle or merciful, with cousins across the Germanic family — German 'mild,' Dutch 'mild' — all tracing back to a Proto-Germanic root suggesting softness or kindness. Some scholars link it further back to a Proto-Indo-European root meaning 'soft' or 'tender,' the same gentle soil that may have grown words for melting and yielding. The '-ness' is the workaday English suffix that turns a quality into a thing you can name, so 'mildness' is simply 'the state of being milde' — softness made into a noun.