1885, in the criminal psychology sense, 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 the means of acquittal, were briefly notorious in England and brought the word into currency in the modern sense.