means The quality of being no longer fresh, crisp, or interesting — whether bread gone hard, air gone musty, or ideas gone tired.

from From 'stale,' which entered English in the Middle Ages with the sense of something settled or aged — likely from an Old French or Anglo-Norman word for liquor that had stood long enough to clear and strengthen (a positive sense at first, oddly). It may share roots with words meaning 'to stand still,' and over time the meaning soured: standing still stopped being a virtue and became the very thing that spoils. The abstract noun 'staleness,' stacking the '-ness' suffix on top, simply names the condition itself.