The expression first appeared in print via the title of a 1920 book, Die Freigabe der Vernichtung Lebensunwerten Lebens (Allowing the Destruction of Life Unworthy of Life) by two professors, the jurist Karl Binding (retired from the University of Leipzig) and psychiatrist Alfred Hoche from the University of Freiburg. According to Hoche, some living people who were brain damaged, mentally retarded, autistic (though not recognized as such at the time), and psychiatrically ill were "mentally dead", "human ballast" and "empty shells of human beings". Hoche felt killing such people was useful. Some people were simply considered disposable. Later the killing was extended to people considered 'racially impure' or 'racially inferior' according to Nazi thinking. This project was extended (in the same killing centers) to "impaired" inmates of concentration and extermination camps and, finally, to mass killings in the extermination camps themselves.

who you going to believe, your self, or the nazis who has in his wikipedia

was a German psychiatrist well known for his writings about eugenics and euthanasia.