E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
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.