社会の窓 (shakai no mado) literally means “a window of the society”, but actually refers to “a trouser fly” in a humorous way. Nowadays, it is regarded as an old-fashioned expression.
It is usually used as “社会の窓が開(あ)いてますよ” (“Your window of the society is open”) when you tell a man humorously that his fly is open.
It is said that this word originates from the Japanese radio show called “インフォメーションアワー・社会の窓“, which had been on air during 1948-1960. This show mainly talked about hidden aspects/problems of the society, and that’s why people started to use the word “社会の窓” to indicate that someone’s fly is open and his hidden thing is at the risk of being exposed to the society.
ファスナー, チャック “a trouser fly”