I always heard that Cirno is the 'strongest' and '9'.
Could someone explain the joke completely?
Phantasmagoria of Flower View's manual had Cirno in a screenshot marked on the accompanied legend as ⑨バカ (9. Stupid), which resulted in ⑨ becoming a fan nickname for her.
It isn't that much of a "fan nickname" now, but rather an overused and heavily abused meme, in other words, cancer. So if you're going to use it, be careful, primaryfags are being very sensitive for this thing lately and you might singe your feathers.