From an earlier neente, either from Latin ne gentem (“no person, no one”), nec entem, ne entem, or ne inde. Compare Old Catalan nient, Sicilian nenti, Piedmontese gnente, Occitan neient, French néant.
The regional pronunciation with initial self-gemination of /nj-/ is by speakers who try to correct the /ɲ-/ because of the orthography, but keep the syllable structure, and thus gemination, of the original /ɲ-/.