The expression first occurs in the title of a 1920 book, Die Freigabe der Vernichtung Lebensunwerten Lebens, (Release for Annihilation of Life Unworthy of Life) by Karl Binding and Alfred Hoche.
The concept culminated in Nazi extermination camps, instituted to systematically murder those who were unworthy to live, according to Nazi ideologists. It also justified various human experimentation, and eugenics programs, as well as racial policies.
According to the author of Medical Killing and the Psychology of Genocide psychiatrist Robert Jay Lifton, the policy went through a number of iterations and modifications:
"Of the five identifiable steps by which the Nazis carried out the principle of "life unworthy of life," coercive sterilization was the first. There followed the killing of “impaired” children in hospitals; and then the killing of “impaired” adults, mostly collected from mental hospitals, in centers especially equipped with carbon monoxide gas. 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."