psychopath n.

1885, in the criminal psychology sense, "a morally irresponsible person," considered as mentally deranged; "one who obeys his impulses regardless of social codes," a back-formation from psychopathic.

The Daily Telegraph had, the other day, a long article commenting on a Russian woman who had murdered a little girl. A Dr. Balinsky prevailed upon the jury to give a verdict of acquittal, because she was a "psychopath." The Daily Telegraph regards this term as a new coinage, but it has been long known amongst Spiritualists, yet in another sense. [The Medium and Daybreak, Jan. 16, 1885]

The case alluded to, and Balinsky's means of procuring the acquittal, were briefly notorious in England and brought the word into currency in the modern sense.