assimilationist (n.)

early 15c., "act of assimilating," in reference to the body's use of nutrition, from Old French assimilacion, from Latin assimilationem (nominative assimilatio) "likeness, similarity," noun of action from past-participle stem of assimilare "to make like" (see assimilate).

The meaning "process of becoming alike or identical, conversion into a similar substance" is from 1620s. The figurative use is from 1790. The psychological sense is from 1855.

word-forming element meaning "one who does or makes," also used to indicate adherence to a certain doctrine or custom, from French -iste and directly from Latin -ista (source also of Spanish, Portuguese, Italian -ista), from Greek agent-noun ending -istes, which is from -is-, ending of the stem of verbs in -izein, + agential suffix -tes.

Variant -ister (as in chorister, barrister) is from Old French -istre, on false analogy of ministre. Variant -ista is from Spanish, popularized in American English 1970s by names of Latin-American revolutionary movements.