1650s, "mix (a metal) with mercury," a back-formation from amalgamation, or else from obsolete adjective amalgamate (1640s) from amalgam (q.v.). Originally in metallurgy; figurative transitive sense of "to unite" (races, etc.) is attested from 1802; intransitive sense "to combine, unite into one body" is from 1797. Related: Amalgamated; amalgamating. Earlier verbs were amalgam (1540s); amalgamize (1590s).