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There are a number of folk etymologies, many based on acronyms — Not Available For Fucking, Normal As Fuck — though these are backronyms.

More likely etymologies include northern UK dialect naffhead, naffin, or naffy, a simpleton or blockhead ; niffy-naffy, inconsequential, stupid, or Scots nyaff, a term of contempt for any unpleasant or objectionable person.

An alternative etymology may lie in the Romany, itself rooted in násfalo, meaning ill.