'assimilationist' originates from modern English, formed from the noun 'assimilation' plus the agentive suffix '-ist', where 'assimilation' comes from Latin 'assimilatio' meaning 'a making similar' (from 'assimilare').

'assimilation' came into English via Late Latin 'assimilatio' (from 'assimilare') and ultimately from Latin elements 'ad-' (to/toward) + 'similis' (like); the modern English formation 'assimilationist' was created by adding the suffix '-ist' to denote a person or tendency.

Initially the root referred broadly to 'making similar' or 'becoming like'; over time the term has come to denote specifically the social or cultural process of minority groups taking on the characteristics of a dominant group, and 'assimilationist' now commonly denotes a person or policy that favors that process.