'assentiveness' originates from Latin via Old French and Middle English, specifically the Latin word 'assentire' (from the elements 'ad-' + 'sentire'), where 'ad-' meant 'to' and 'sentire' meant 'to feel' or 'to perceive'.

'assentiveness' changed from Middle English forms related to 'assenten' and Old French 'assentir', derived from Latin 'assentire'; the modern English noun was formed by adding the adjectival suffix '-ive' and the noun-forming suffix '-ness' to the base 'assent'.

Initially it meant 'to feel toward' or 'to express assent' (from the Latin root), but over time it evolved into the current sense of 'a readiness or tendency to agree'.