The story
Uncertain. The English Dialect Dictionary mentions several other spellings found in other dialects (nedack, neddick, nudack, nuddick, nudeck) and compares it to nod (“nape of the neck”), found in e.g. John Coker Egerton's Sussex Folk and Sussex Ways (1884). Compare the ending to -ock.
Etymology from Wiktionary, CC BY-SA 4.0.