(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.
Don't be such a ⑨!
by redtails April 14, 2010
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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.
Normal high schooler: Oh, hey.
Weird-ass weeb: Oh wassup ⑨⑨⑨
by Monika from DDLC but sane September 11, 2019
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