'severally' originates from Middle English, specifically formed from the adjective 'several' with the adverbial suffix '-ly'; 'several' itself comes from Old French 'several' (or Medieval Latin 'separalis'), where the root 'separ-' (from Latin 'separare') meant 'to separate'.

'severally' changed from Middle English usage derived from Old French/Medieval Latin forms (such as Old French 'several' and Medieval Latin 'separalis') and eventually became the modern English adverb 'severally' by adding '-ly' to 'several'.

Initially it meant 'in separate or separate ways' (derived from the idea of 'separating'); over time it retained essentially the same sense of 'individually' or 'separately' in modern usage.