Hello Reg Bowler,
Thank you for getting in touch. I must say, I was surprised by your message, as I have never heard the term nig-nog as anything other than an extremely offensive racist term, and I am also from the UK. However, I have checked and found this definition in the OED: "A new or unskilled recruit; a novice; a foolish or naive person."
Could I ask what part of the country you are from and whether this sense of the term is still in common use there?