“Nom de guerre ” is French for a “war name,” but really means an assumed name. It was customary at one time for everyone who entered the French army to assume a name, this was especially the case in the times of chivalry, when knights went by the device of their shields or some other distinctive character in their armour, as the “Red-cross Knight.”
“Nom de plume. ” English-French for the “pen name,” and meaning the name assumed by a writer who does not choose to give his own name to the public, as Peter Pindar, the nom de plume of Dr. John Wolcot; Peter Parley, of Mr. Goodrich, Currer Bell, of Charlotte Brontë; Cuthbert Bede, of the Rev. Edward Bradley, etc.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Nom from Infoplease: