"interbreeding of races," applied originally and especially to sexual union between black and white individuals, 1863, coined irregularly by U.S. journalist David Goodman Croly from Latin miscere "to mix" (from PIE root *meik- "to mix") + genus "race," from PIE root *gene- "give birth, beget," with derivatives referring to procreation and familial and tribal groups. It first appeared in "Miscegenation: The Theory of the Blending of the Races, Applied to the American White Man and Negro," a pretended anti-Abolitionist pamphlet Croly and others published anonymously in advance of the 1864 U.S. presidential election. The old word was amalgamation.
The design of "Miscegenation" was exceedingly ambitious, and the machinery employed was probably among the most ingenious and audacious ever put into operation to procure the indorsement of absurd theories and give the subject the widest notoriety. The object was to so make use of the prevailing ideas of the extremists of the Anti-Slavery party, as to induce them to accept doctrines which would be obnoxious to the great mass of the community, and which would, of course, be used in the political canvass which was to ensue. [P.T. Barnum, "The Humbugs of the World," 1866; he also writes that, despite the pamphlet being an ingenious and impudent literary hoax, the word "has passed into the language and no future dictionary will be complete without it."]