'assume' originates from Latin, specifically the word 'assumere', where the prefix 'ad-' (appearing as 'as-') meant 'to/toward' and 'sumere' meant 'to take'.

'assume' changed from Latin 'assumere' into Old French (as 'assumer') and Medieval/ Middle English forms (e.g. 'assumen'), eventually becoming the modern English 'assume'; the adverb 'assumably' is formed from 'assume' + '-ably/-ably' development in post-medieval English.

Initially it meant 'to take up or take to oneself' (literally 'to take'), but over time it evolved into the current common sense 'to take for granted, suppose, or suppose to be true'.