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.
"one who advocates racial or ethnic integration," 1900, originally in reference to Hawaii and possessions obtained by the U.S. in the war against Spain; later with reference to Jews in European nations; see assimilation + -ist. In Portuguese, assimilado (literally "assimilated," past participle of assimilar) was used as a noun of natives of the Portuguese colonies in Africa who were admitted to equal rights and citizenship.
"a making human or humane; assimilation to humanity," 1753, also humanisation, noun of action from humanize.
before vowels quinqu-, word-forming element from classical Latin meaning "five, consisting of or having five," from Latin quinque "five" (by assimilation from PIE root *penkwe- "five").
1590s, "a swallowing up" (now obsolete), from Latin absorptionem (nominative absorptio) "a swallowing," noun of action from past-participle stem of absorbere "swallow up" (see absorb). From 1714 specifically of "disappearance by assimilation into something else."