'assumptiveness' is a Modern English formation from the adjective 'assumptive' plus the nominalizing suffix '-ness' (creating a noun meaning 'the state or quality of being assumptive').

'assumptive' and 'assumption' derive from Latin 'assumere' (ad- + sumere 'to take'); Latin 'assumptio' gave Old French/Latin forms such as 'assumption', which entered Middle English and evolved into modern words like 'assume', 'assumption', 'assumptive' and then 'assumptiveness'.

Originally the Latin root conveyed 'to take to oneself' or 'to take up'; over time English senses shifted toward 'taking something for granted' or 'forming a conclusion without proof', which is the sense behind 'assumptiveness'.