"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)