'assimilative' originates from Latin, specifically the verb 'assimilare', where 'ad-' (in assimilare the 'd' assimilated to 's') meant 'to' or 'toward' and 'similis' meant 'like'.

'assimilare' passed into Late Latin and formed the Medieval/Modern Latin 'assimilare' and the English verb 'assimilate' (via Middle English), from which the adjective 'assimilative' was formed by adding the adjectival suffix '-ive'.

Initially, the Latin root meant 'to make similar' (literally 'to make like'), and over time this sense broadened into the modern uses: describing processes that make things similar (phonetic assimilation) or that absorb/integrate elements (social/cultural assimilation).