Wonderhoy

A word made up and mainly used by Emu Otori, a fictional character from the mobile game "Project Sekai".
Although she mostly uses the word as a greeting to people, she also often uses it as an adjective, some of her troupe members signaling her that it didn't make sense used as such.
She also uses it as a simple sign of her happiness, like in the songs "Niccori^^ Survey Team", "Positive Dance Time" and "Glory Steady Go!".

The phrase quickly became a meme in the Project Sekai community, and many use it among fellow fans as a greeting.
"Wonderhoy" became popular because it is said quite a lot in the game, not only by Emu herself but also by the others in Wonderlands×Showtime, by some of her other friends/classmates and by the Vocaloids.

Some people also spell it "wandahoy" or "wondahoy"

Minori: Good morni-
Emu: KONNICHI-WONDERHOY!! :D
Minori: *blinded by the Emu power*