“Lest we forget” is a phrase that is commonly used in remberance of the millions of men who died in the Great War. It is difficult to describe exactly what it means, but it is a warning of sorts. A warning that we should not forget the soldiers who fought in WWI, and what they endured.
Person 1: “The 100th year anniversary of the end of WWI is on 11/11/2018.”

Person 2: “Lest we forget...”
by A solid cube of tungsten October 25, 2018
Lest We Forget is the Best Of album of Marilyn Manson.
Bob- I'm gonna go buy "Lest We Forget."
Zacharias- What's that?
Bob- "Lest We Forget" is Marilyn Manson's best of album.
Zacharias- Oh that freak?
Bob- Hey!! Listen and Learn before you judge you little prick.