Least, pronounced [LEEST], is the superlative of the adjective little: little, littler, least. It can also function as noun and adverb:
She passed the exam without the least preparation. (adjective)
Truly, I say to you, as you did it to one of the least of these my brothers, you did it to me. (noun)
The person who can least spare it is often the most willing to give others a piece of his mind. (adverb)
Lest, pronounced [LEST], is a conjunction that introduces a clause that expresses something that should be guarded against. Although still used by modern writers and speakers, it has a distinctly literary flavor:
Give not that which is holy unto the dogs, neither cast ye your pearls before swine, lest they trample them under their feet, and turn again and rend you. Matthew 7:6
Beware lest you lose the substance by grasping at the shadow. (Moral to Aesop’s tale of “The Dog and the Shadow.”)
The formulaic phrase “lest we forget” is especially popular at the time of year when a nation honors its military forces and commemorates the slain. Used this way, it means “So we don’t forget,” or “Let us not forget.” For example, this past Memorial Day the phrase appeared on hundreds of websites above photo collections, memoirs, and poems dedicated to the memory of enlisted men and women who gave the supreme sacrifice.
“Lest we forget” is also used as a cautionary expression, warning of disappointment or danger to ensue if something is forgotten:
Lest we forget the past…Buying broken or unfinished games.
Lest We Forget. “Remembering Abbott’s Past”
Lest we forget, child abuse has many forms.
Unfortunately, a great many English speakers are confused about the difference between least and lest–at least when it comes to the phrase “Lest we forget.” A Web search brings up hundreds, perhaps thousands of sites headed by the phrase “Least We Forget.” Here are a few examples that go beyond the three words:
Least we forget in the Twenty First Century
Least we forget the Holocaust. Least we forget Vietnam, Ethiopia, you insert any war you like.
Least we forget what China actually is…
I even found a song title spelled “Least We Forget.”
In conclusion, lest you forget, lest rhymes with west.