C19: perhaps alteration of obsolete English dialect nigmenog silly fellow, of unknown origin
nong in American English
(nɑŋ)
noun
Austral & NZ informal
a foolish, incompetent person
Word origin
[1940–45; prob. by shortening of Australian and earlier Brit. slang ning-nong, ning-nang fool, perh. expressive vars. of earlier nigmenog fool, of obscure orig.]