late 14c., "to make wealthy," from Old French enrichir "enrich, enlarge," from en- "make, put in" (see en- (1)) + riche "rich" (see rich). Figurative sense "supply with abundance of something desirable" is from 1590s. Meaning "to fertilize" is from c. 1600. Scientific sense of "to increase the abundance of a particular isotope in some material" is first attested 1945. Related: Enriched; enriching.
