nixie

Etymology 1

From German Nixe, feminine of Nix, from Middle High German nickes, from Old High German nihhus (“water-elf, crocodile”), from Proto-Germanic *nikwus, *nikwis, perhaps from Proto-Indo-European *neygʷ- (“to wash”). Cognate with Old English nicor (“water-elf, hippopotamus, walrus”), English nicker.

noun

1. A female nix, a water-spirit.
   The place seems to have been fashioned as a dwelling for dryads and hamadryads, for nixies and pixies, and all the fabled spirits of forest and stream. 1916, Rex Beach, Rainbow's End, published 2008, page 1
   In medieval times, the image of the double fish tailed siren was called a nixie or evil water spirit. Yet, the nixies were more likely daughters of the primordial goddess named Nyx or mother night, who hovered over ancient waters, rather than evil spirits. 2011, Katalin Koda, Fire of the Goddess, page 83

Etymology 2

From nix + -ie.

noun

1. A piece of mail returned as undeliverable.
   1994 March 3, United States Postal Service, Postal Bulletin, page 4, Mailers who are registered participants in the Postal Service's Address Change Service (ACS) system expect to receive their address corrections and nixie notifications by electronic messages; […] .
   If you get a response of less than one percent or excessive “nixies” (wrong addresses), rework your copy or get a new list. 2003, Jenkins Group, The Insiders Guide to Large Quantity Book Sales, page 7
   Another good reason to use first-class mail is the fact that it will often be forwarded or, if need be, returned to the sender if the address is bad. (In the industry, this is called a “nixie.”) It is important to keep an eye on the nixie rate if you are using first-class mail. 2008, Susan K. Jones, Business-to-Business Internet Marketing, 5th edition, page 15