1650s, "mix (a metal) with mercury," a back-formation from amalgamation, or else from the obsolete adjective amalgamate (1640s) from amalgam (q.v.). Originally in metallurgy. The figurative transitive sense of "to unite" (races, etc.) is attested from 1802; the intransitive sense of "to combine, unite into one body" is from 1797. Related: Amalgamated; amalgamating. Earlier verbs were amalgam (1540s); amalgamize (1590s).
1610s, "act of compounding mercury with another metal," noun of action from archaic amalgam (v.) "to alloy with mercury" (see amalgamate). The figurative, non-chemical sense of "a combining of different things into one uniform whole" is attested from 1775. Especially of the union or merger of corporations under one direction.