1828, in part a back-formation from pseudonymous, in part from German pseudonym and French pseudonyme (adj.), from Greek pseudonymos "having a false name, under a false name," from pseudes "false" (see pseudo-) + onyma, Aeolic dialectal variant of onoma "name" (from PIE root *no-men- "name").
"Possibly a dictionary word" at first [Barnhart]. Fowler calls it "a queer out-of-the-way term for an everyday thing." Properly in reference to made-up names; the name of an actual author or person of reputation affixed to a work he or she did not write is an allonym. An author's actual name affixed to his or her own work is an autonym (1867).