⑨

Cirno

(9) or nine-ball.

In the game manual of Touhou's 9th game (東方花映塚 ~ Phantasmagoria of Flower View), Cirno, pitted as the second player character, was labeled as "⑨" - it stood for バカ ("baka" - "idiot"). Ever since, ⑨ gained notoriety as Cirno's nickname.

A word that is an inside joke for those who know what it means, even though nobody actually knows what it means. It’s basically a nine in a circle for a normie, but its meaning changes if you are a senpai or higher.