'autumnally' is formed in English from the adjective 'autumnal' plus the adverbial suffix '-ly'; 'autumnal' ultimately comes from Latin 'autumnalis', from 'autumnus' meaning 'autumn'.

'autumn' came into English via Old French 'autompne/automne' from Latin 'autumnus'; 'autumnal' comes from Latin 'autumnalis' and English formed 'autumnally' by adding '-ly' to create the adverb.

Originally describing something 'of or relating to autumn' (adjective), the derived adverb 'autumnally' has the specific meaning 'in a manner characteristic of autumn'.