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1.
The sound of a cat in Japanese. "Nyaa (ニャー)" = "Meow".
Cat: Nyaa Nyaa! (ニャー ニャー!)
by Shironeko August 28, 2012
 
2.
An exclamation a girl makes when happy, hyper, excited or horny
"Nyaa!! I'm so happy!"
by Jacks-sis13 January 31, 2009