question...

What the hell does nyaa~ mean
well i know it's an expression, but for what use?
thank you in advance.

its what a cat says

That made you my favorite person for the day.

But anyway, essentially it's a speech equivilent of the sounds a cat makes. basically a Japanese "meow" I can't recall what term is used to describe when you do that, but hey.

so pussy cat says nyaa?

Yep.

Next time your cat thinks it's alone, it'll actually say that.

Basically

Asian girls say it in place of more coherent, intelligent speech.

thanks anyways