An Etymological Dictionary of the German Language/Nix
Nix, m., ‘nixey, water-fairy,’ from MidHG. (very rare) nickes, OHG. nihhus, n. and m., ‘crocodile’; comp. AS. nicor, ‘hippopotamus,’ E. nick, ‘water-sprite’ (Old Nick, applied to the devil), MidDu. nicker, ‘water-sprite,’ OIc. nykr (from *niqiza), ‘water-sprite in the form of a hippopotamus,’ also ‘hippopotamus.’ The OHG. and MidHG. sense ‘crocodile’ is easily associated with the other meanings of the cognates; the prim. signification may be ‘fabulous sea-monster.’ The word is probably based on a Teut. root niq from pre-Teut. nig (Sans. nij, Gr. νίπτω), ‘to wash oneself’; thus Nix would mean orig. ‘a sea-animal that delights in bathing, sea-spirit,’ while the masc. Nix, like AS. nicor, points to Goth. *niqiza, *nikuza-, the corresponding fem. Nixe, preserved only in HG., indicates Goth. *niqisi; OHG. nicchessa, MidHG. *nickese, *nixe, in waȥȥernixe, f., ‘female water-sprite,’ for which in MidHG. męrwîp and męrmeit are used.