*nun'yah; contraction of "none of your"
Slang expression originating in the southern United States and one of those bits of localized slang that are spreading swiftly due to the internet; its use is currently widespread among the 12-25 age group and it's frequently found in use on the internet as a response to questions posed in online forms, forums and discussion groups or elsewhere as a generic semi-serious reply. Some people seem to be using the word to create fictitious names that will confuse those unfamiliar with it (e.g. "My name is 'Nunya'" or "Nunya Lake").
The "none of your" it implies is "business" ("nun ya bizness" being its original form). Its implied meaning is "none of your damned business and what's it to you anyway?" Its impolite implications place it strictly in the 'slang' category though it wouldn't surprise me if it emerged as a common colloquialism one day.