French for "name" (9c.), from Latin nomen (see nominal). It is used in various phrases in English, such as nom de guerre (1670s) "fictitious name used by a person engaged in some action," literally "war name" and formerly in France a name taken by a soldier on entering the service, and nom de théâtre "stage name" (1874). Nom de plume (1823) "pseudonym used by a writer," literally "pen name," is a phrase invented in English in imitation of nom de guerre. Fowler suggests it is "ridiculous for English writers to use a French phrase that does not come from France."
Latin adverb and preposition of separation in space, meaning "down from, off, away from," and figuratively "concerning, by reason of, according to;" from PIE demonstrative stem *de- (see to). Also a French preposition in phrases or proper names, from the Latin word.
active word-forming element in English and in many verbs inherited from French and Latin, from Latin de "down, down from, from, off; concerning" (see de), also used as a prefix in Latin, usually meaning "down, off, away, from among, down from," but also "down to the bottom, totally" hence "completely" (intensive or completive), which is its sense in many English words.
As a Latin prefix it also had the function of undoing or reversing a verb's action, and hence it came to be used as a pure privative — "not, do the opposite of, undo" — which is its primary function as a living prefix in English, as in defrost (1895), defuse (1943), de-escalate (1964), etc. In some cases, a reduced form of dis-.
also deaccession, "remove an entry for an item from the register of a museum, library, etc." (often a euphemism for "to sell"), by 1968, from de- "off, away" + accession, which had been used since 1887 in library publications as a verb meaning "to add to a catalogue." Related: De-accessioned; de-accessioning.