'assumingness' ultimately derives from Latin 'assumere', where the prefix 'ad-' (later assimilated to 'as-') meant 'to/toward' and 'sumere' meant 'to take'.

'assumere' passed into Medieval Latin and Old French as 'assumer' and entered Middle English in forms like 'assumen'/'assume', later developing into the modern English verb 'assume' from which the deverbal noun formation '-ing' + '-ness' produced 'assumingness'.

Originally the Latin root meant 'to take up' or 'take to oneself'; over time the sense shifted toward 'to suppose' or 'to take for granted', and 'assumingness' captures the resulting notion of taking things for granted or being presumptuous.