What means ,,Nyappy'' ?

Nyappy is the "logo" of the band. Miku-san сказал(-а) that Nyappy was created like "Happy" and unnyappy is "Unhappy", but with time, last год (2009), they сказал(-а) it was used for everything (answering the phone, bye, hello, how are you, etc.)

What Miku is doing with the hand is the "nyappy"

NYAPPY means hello или cool an cafe style