@JTT,
JTT wrote:It is an English phrase.
Well, I suppose you could say it is an "imported" French phrase, but I wonder how you explain away the circonflexe? English contains many words of French origin which are pronounced according to English rules of phonology, rather than French. Around 28% of English vocabulary is of French or
langues d'oïl origin, most derived from, or transmitted by, the Anglo-Norman spoken by the upper classes in England for several hundred years after the Norman Conquest, before the language settled into what became Modern English. However I take the view that
raison d'être should be classified among those words or phrases that generally entered the lexicon later, e.g. through literature, the arts, diplomacy, and other cultural exchanges not involving conquests. As such, they have not lost their character as Gallicisms, or words that seem unmistakably foreign and "French" to an English speaker.