means The emotional state of unhappiness, sorrow, or low spirits, usually in response to loss, disappointment, or pain.

from From the old word 'sad,' which in Old English (sæd) didn't mean unhappy at all — it meant 'sated,' 'full,' 'weary,' a cousin of 'satisfied' and Latin satis ('enough'). Over centuries the sense drifted from 'full to the point of heaviness' to 'settled and serious' to, finally, 'sorrowful' — as if too much of life had weighed the spirit down. The '-ness' is the plain English suffix that turns a quality into a thing you can name and carry.