Green’s Dictionary of Slang

nig-nog n.1

also nig
[The Times (30 November 1967) claims the term was used ‘long before coloured immigrants appeared...’ but the greater likelihood is that the stereotype of the incompetent black labourer was the real origin, although note nigmenog n. + Scot. nig-nag, a worthless, useless thing]

1. (orig. railway) a novice, an unskilled person.

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[UK]Punch 9 Dec. 692/3: All must be represented on a strict basis of proportion of the number of citizens for whom they cater: Football-pool promoters (six representatives), barrow-boys (two representatives), share-pushers, erks, nig-nogs, [etc.] .
[UK]C. Wood ‘Prisoner and Escort’ in Cockade (1965) I ii: Nig-nog games – don’t shame me ’Oskie. [Ibid.] I iii: Little nig nogs wrapped in denims / Mean more to me than Naafi tea [...] They’ll be soldiers / Some of the time.
[UK]C. Wood ‘Spare’ in Cockade (1965) I ii: He don’t take no notice of nigs.
[UK]B.S. Johnson All Bull 92: I felt sick and miserable standing in the long queue of ‘nig-nogs’ [i.e Army recruits].

2. a fool, a simpleton.

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[UK]A. Wesker Chips with Everything I iii: A straight line, you heaving nig-nogs, a straight line.
[Aus]A. Chipper Aussie Swearers Guide 43: A ning-nong is a nig-nog is a dill is a galah is a boofhead is a drongo is a ning nong ning. Just another snappy Aussie euphemism for for that thoughtless bird-brained idiot next door.