Palladium
The most common usage is in reference to a valuable metal used in electronics and catalytic convertors. It was discovered in 1803.
However, an idiomatic usage is for something that affords protection. It comes from “Pallas,” which was another name for the Greek goddess Athena. A statue of Athena was believed to have the power to protect the city of Troy.
Merriam-Webster provides some examples of this usage:
…this political persecution being, somehow or other, the grand palladium of our liberties.
…lend authority to the view that the true palladium of every people is its language…
I had to search for “palladium idiom” to find this usage, so, clearly, a reference to the metal is much more common.
Why I Looked It Up
In his farewell address, George Washington said this:
It is of infinite moment that you should properly estimate the immense value of your national Union to your collective and individual happiness; that you should cherish a cordial, habitual and immoveable attachment to it; accustoming yourselves to think and speak of it as of the Palladium of your political safety and prosperity…
Postscript
Added on
I was in the London Underground when I saw a poster advertising a concert at The London Palladium, which is a historic theater. I then remembered that’s there’s a Palladium in Hollywood as well.
Sadly, I couldn’t find any etymological information about either of them. The one in Hollywood opened in 1940, and the one in London opened in 1910.
As of right now, I can’t determine why either of them were named “Palladium.”