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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