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]]
114514 (meme)