《汉书·志·五行志下之上》
一说，有脂物夜降为妖，如果脂水夜降污染了衣物，就是淫乱的象征。
一曰，有脂物而夜为妖，若脂水夜污人衣，淫之象也。
Some say that there is fat in the night descending as a demon. If the fat water descends in the night and defiles the clothes, it is a symbol of promiscuity.