nixie n.

"water fairy, water sprite," 1816 (introduced by Sir Walter Scott), from German Nixie, from Old High German nihhussa "water sprite," fem. of nihhus, from Proto-Germanic *nikwiz (source also of Old Norse nykr, Old English nicor "water spirit, water monster," also used to gloss hippopotamus), perhaps from PIE *neigw- "to wash" (source also of Sanskrit nenkti "washes," Greek nizo "I wash," Old Irish nigid "washes").