'autumnity' originates from Latin, specifically the word 'autumnus', where 'autumn-' meant 'autumn' and the suffix '-itas' (from which English '-ity' derives) meant 'state or quality'.

'autumnity' formed in English by adding the Latin-derived suffix '-ity' to Middle English 'autumn' (from Old French/Latin roots), producing a noun meaning the state or quality of autumn; it appears as a rare/archaic formation in Early Modern English and later usage.

Initially it meant 'the state or quality of autumn', and over time this basic meaning has been retained though the word became rare and is now largely archaic or poetic in use.