'assortative' is formed from the verb 'assort' plus the adjective-forming suffix '-ative'. 'assort' comes from Old French 'assortir' (to arrange, agree) ultimately through Latin elements such as the prefix 'ad-' (to) and 'sors/sort-' (lot, fate).

'assortir' passed into Middle English as 'assort' meaning 'to arrange or match'; in Modern English the suffix '-ative' (from Latin '-ativus') was added to produce 'assortative' meaning 'tending to assort or match'.

Initially related to 'arranging' or 'making compatible', it evolved to describe a tendency or characteristic of matching or pairing (especially in biological contexts like mating patterns).