I knew the word Perfidious from its best use, Perfidious Albion.
That's one use of the word that was fucking earned. Don't trust the word of the British Empire.
It's also the only use that should really exist anymore. Anyone who uses perfidious in a sentence that is not criticizing the British needs to just stop. And if they're a Nazi jackass, they need to take a thesaurus and slam it repeatedly against their own head until they stop being a Nazi.