Can be used in a variety of contexts, most often to express dissatisfaction, or that you think whatever you're talking about is silly or stupid. Can also be used mid-sentence, for example "I gammon told her that she is a frog" or "I was just gammoning!" indicate that what was said, was said in a joking manner. In wide use in the Northern Territory of Australia.
Definitions include: Euphemism for vagina. Compare with whispering eye "Had myself a juicy morning orange last night" "Yeah, ended up in a thigh choke hold till I passed out last night after I bit into a delicious morning orange myself. Just couldn't help it."