The erotic sense traces to Song Yu's 《
高唐赋》 (Gāotáng Fù, c. 3rd century BCE): King Huai of Chu dreams of a goddess on Mount Wu (
巫山, Wūshān) who tells him she is 'morning clouds, evening rain' (
旦为朝云,
暮为行雨). Ever since,
巫山云雨 (Wūshān yúnyǔ) has been the classical Chinese euphemism for lovemaking — softer and more poetic than direct vocabulary.