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"water fairy," 1816 (introduced by Sir Walter Scott), from German Nixie, from Old High German nihhussa "water sprite," fem. of nihhus, from Proto-Germanic *nikwiz (cf. Old Norse nykr, Old English nicor "water spirit, water monster," also used to gloss hippopotamus; Grendel's mother in "Beowulf" was a nicor), perhaps from PIE *neigw- "to wash" (cf. Sanskrit nenkti "washes," Greek nizo "I wash," Old Irish nigid "washes").
A piece of mail that cannot be delivered because of damage, illegibility, etc (1885+ Post office)
in Germanic mythology, a water being, half human, half fish, that lives in a beautiful underwater palace and mingles with humans by assuming a variety of physical forms (e.g., that of a fair maiden or an old woman) or by making itself invisible. One of three attributes may betray the disguises of nixes: they are music lovers and excellent dancers, and they have the gift of prophecy. Usually malevolent, a nix can easily be propitiated with gifts. In some regions, nixes are said to abduct human children and to lure people into deep water to drown. According to some sources, nixes can marry human beings and bear human children.