Fowler/The King's English asserts that "nom de plume" was invented in English to "correct" the easily misapprehended "nom de guerre" (also, "nom de plume" was used in English before "pen name") and was presumably borrowed back by the French, who didn't themselves use the term.
Comments
and the legendary mightiness of one over the other…
As a French, I find the word "maitre d'" very funny because the hotel part of the phrase (maître d'hôtel) just disappeared, and the ' remained. It's like saying "King o" instead of "King of England."