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). Meaning "process of becoming alike or identical, conversion into a similar substance" is from 1620s. Figurative use from 1790. Psychological sense is from 1855.
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