'assemblagist' originates from French 'assemblage' (a gathering or putting together), formed in English by adding the agentive suffix '-ist' to denote a practitioner.

'assemblage' entered English from French in the 19th century as both a general term for a gathering and later as an art term; 'assemblagist' was later coined in English to mean 'one who makes assemblages.'

Initially 'assemblage' meant a collection or gathering; over time it acquired a specialized art-technical sense as a work composed of assembled objects, and 'assemblagist' came to mean a maker/practitioner of such works.