Also known as: 114514
A popular meme from Manatsu No Yo No Inmu that made its way to Cookie (Touhou) as well.
From comment #2010702:
Like all numbers in this picture, it's Japanese number code.
1 = i(chi)
4 = shi/yo
5 = go/ko
So it can be translated as 'iiyo, koi yo' or 'all right, cum!'
Other related numbers that sometimes appear as well:
1919: Saying each number invidually becomes "i ku i ku", which sounds like "cum, cum"
810: Can be split into "8 10", then translated as "ya ju", for Yajuu Senpai
3777: Sa(n) na(na) na(na) na(na) = Sananana (Cookie)
364364: 見ろよ、見ろよ, said by Miura Daisenpai