What does ‘perfidiously’ mean?

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Increase your vocabulary and you’ll make your writing much more precise. That’s why I provide a word of the week. Today’s word: perfidiously…

I’ve never been much of a fan of murder mysteries or spy fiction, but I’ve read some John Le Carré (pictured above), and his work impresses me. A former spy — who worked for the British  Security Service and the Secret Intelligence Service — Le Carré achieved fame as a writer with his 1963 bestseller The Spy Who Came in from the Cold

Born in 1931 in Poole, England, as David John Moore Cornwell, the author has written some 23 novels, three books of non-fiction and three screenplays.

A story in the December 2015 Atlantic, headlined “The Double Life of John Le Carré,” and written by James Parker not only outlines Le Carré’s multi-faceted life (fascinating, even to those uninterested in spy fiction) but also gives me my word of the week, perfidiously. Here’s how Parker used it:

But not every spy is a writer. Kim Philby, for example, the Soviet double agent who spent a perfidiously productive decade in the highest echelons of Cold War British intelligence, was also responsible for some appalling prose.

The adverb perfidiously refers to someone who behaves in a deliberately faithless, treacherous, or deceitful manner. In short: it is someone who cannot be trusted. The term originates from the Middle French word perfidie, which, in turn comes from the Latin perfidus “faithless,” with per meaning “through,” and fidem meaning “faith.”

I like the way the author chose the word, undoubtedly for the alliteration of the explosive P sound.