they have some form of mental retardation. It is a mild level insult basically.
Jack: Cheese please.
(Dillan puts ham in the sandwich)
Jack: (Upon realising there is ham in the sandwich) Oh Dillan you nonger.
It is also a less serious term, therefore sounding less offensive, and because of the ridiculous sound of the word, it can be taken more lightly.
2. Can be used to refer to the male’s genital
Boy “Did you know I can skull a whole sack of goon in five minutes?”
Girl “That’s not even cool, you nonger!”
Girl “Did you see how big his nonger was?”