'asseveratively' originates from Latin, specifically the word 'asseveratus' / 'asseverare', where 'ad-' meant 'to/toward' and 'severus' meant 'serious'.

'asseveratively' developed from Latin 'asseveratus' (past participle of 'asseverare') into the English verb 'asseverate' and adjective 'asseverative', and the adverb 'asseveratively' was formed from those.

Initially it meant 'to affirm or declare solemnly', and over time it has retained that core meaning as 'in a solemn or emphatic manner.'