Literally, "life unworthy of life". It predates the Nazi regime; the phrase was coined by Karl Binding and Alfred Hoche in a book published in 1920.
John Locke:
Good post, more broadly, both Hoche and Binding were both part of the eugenics movement that started in the late 19th and early 20th century, of which of course Margaret Sanger, the “founder and patron saint” of Planned Parenthood was also part of.
Apparently, one of Hoche’s relatives were later killed by the Nazi euthansia program, which was largely based on his works of the 1920’s, and it is reported that he privately criticized it, ohhh, the liberal double standard.