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1.
A word, popularized by Voldemort, used to express extreme emotion, most often anger or hate.
Harry Potter: "Come on, Tom. Let's finish this the way we started, together." *jumps off cliff*

Voldemort: *as they are falling* "NYEAH!!!!!!!"
by Xeno LoveGood June 20, 2011
 
2.
there is no definition, it can be used in any way
1. Man that is so nyeah!
2. I'm going to nyeah you up so bad
3. NYEAH!
4. i got nyeah'd..
5. Holy nyeah!
by Peter Hwang October 25, 2006
 
3.
A noise Lord Voldemort seems to make quite a lot, usually before falling off a cliff or doing something epic.
Example 1 -
Harry: Come on, Tom. Let's finish this the way we started it. Together!
Voldemort: NYEAH!!!

Example 2 -
Voldemort: Only I can live forever. NYEAH!!!
by skafruit29 August 29, 2011
 
4.
The sound an airplane makes. Can be yelled at quiet moments to freak people out. Can also be used to end awkward silences. Is sometimes followed by "I'm going in!"
*Classroom suddenly grows awkwardly quiet* "...Nyeah!" *Laughter, and strange looks* "Nyeah!" "Nyeah, I'm going in!"
by CodeRandom February 27, 2011
 
5.
A general word used when the intention is to emphasize a taunt or poke fun, tickle, dig, cut, gibe, jab, jeer or otherwise gesticulate derisively.
I have bested you once again, you humper of other peoples dogs! N'YEAH!
by Christopher Gray January 01, 2013
 
6.
Acronym for "not yeah", usually used as a sound of discontent, especially when someone does is not enthusiastic about something another person has said.
Tom: "So what did you think of that movie?"

Sam: "Nyeah, it was okay."
by Big White August 19, 2004