1. A greeting.
2. An exclamation of suprise.
3. An exclamation of shock and/or disgust.
4. A noise
2. Disgust - Your brother tells you he was snorting coke with a friend and said male friend suggested performing fellatio on him. You respond with a prompt and crisp "Nyiii !"
3. War Cry - At a good night out at the pub with the lads many cries of "Nyiii ! " will definitely be heard.
4. General Response to Stupid Shite - Someone says something gayer than a tennis helmet, leaving you with an awkward feeling. The only logical response is a prompt and agressive "Nyiii ! "leaving the culprit feeling like a knob.
It can express enthusism, surprise, dismay, and delight, but whatever the usage, is best shouted with vivacity and extreme obnoxiousness.
Sighting an old friend on the crowded subway platform, Chris got his attention by crowing "NYIII!"
Usages 2 and 3- Surprise, dismay:
Clicking a link he was told would lead to an anticipated movie trailer, Chris was confronted with the horrific visage of Goatse. "NYIII!" he shouted, frantically hammering on the Back button.