A few years ago I was told by a Japanese friend "社会の窓" (shakai no mado).
It was explained after some giggling that this is what is said to a man who has inadvertently left his fly open, and that it means literally "society's window".
So why use "society's window" for this situation? Is it actually an additional sense added to a more straightforward older one? Or does it provide some insight into Japanese culture why such wording would be used?