Nixie

Completely unexpected definition. As an editor and lover and occasionally writer of sci-fi and fantasy, I am familiar with water nixies. Definition from Mostropedia: The Neck, Nixie (English) or 'Nix, Nixe or Nyx (German) are types of river merman and mermaid who may lure men to drown, akin to the Celtic Melusine and similar to the Greek Siren.

Read more: http://monstropedia.org/index.php?title=Nixie#ixzz4EO2tBBrz

I was gob-smacked to discover there was a second (or singular according to dictionary.com) definition:

nixie

\NIK-see\

noun 1. a letter or parcel that is undeliverable by the post office because of a faulty or illegible address.