The story
Current use as slogan popularized after the Holocaust, including by Meir Kahane's titular 1972 book and Jewish Defense League's 1968 adoption of the slogan.
Etymology from Wiktionary, CC BY-SA 4.0.
never again
Today it means: At no time in the future; usually said of a negative event, practice, etc., which should not be allowed to reoccur. Full definition →