'assumptively' originates from Latin, specifically the word 'assumptus', the past participle of 'assumere', where 'ad-' (ass- assimilated) meant 'to/toward' and 'sumere' meant 'to take'.

'assumptively' changed through Medieval Latin and English forms: Latin 'assumptus' led to Medieval Latin 'assumptio' ('assumption'), which passed into Old/ Middle English as 'assumption'; the adjective 'assumptive' was formed in modern English and then the adverb 'assumptively' was created by adding the suffix '-ly'.

Initially, it meant 'taken up' or 'adopted' (in Latin contexts), but over time it evolved into its current sense of 'in a manner based on assumption' in modern English.