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1.
The noise that the Pokemon "Staryu" makes
Trainer- Staryu! Use rapid spin!
Staryu- Hyah!
by CalvinG August 12, 2009
 
2.
An exclamation of approval or excitement. Can also be used as a greeting an exclamation of triumph or as a way to show confusion about what someone has said.
"Cake is on Conan tonight! Hyah!"

"HYAH! ...Bitches"
by PfVegas December 06, 2010