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1.
A 'silly' person. It is used as a more harsh form of 'idiot' but not quite as offensive as implying they have some form of mental retardation. It is a mild level insult basically.
Dillan: Do you want cheese or ham in your sandwich?

Jack: Cheese please.

(Dillan puts ham in the sandwich)

Jack: (Upon realising there is ham in the sandwich) Oh Dillan you nonger.
by Jack (JAM) April 11, 2008
 
2.
1. A less harsh word for fuckwit, dick, idiot, and all other offensive terms refering to someone who is being silly.
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
E.g. 1
Boy "Did you know I can skull a whole sack of goon in five minutes?"
Girl "That's not even cool, you nonger!"

E.g. 2
Girl "Did you see how big his nonger was?"
by bearcakes91 May 14, 2009