“白” means “white,” and “月光” means “moonlight.” Quoted from late Chinese author Eileen Chang’s novel “Red Rose, White Rose,” this term refers to a spotless woman a man cherishes in his memory whom he loved but who is no longer with him. The term caught on with the popularity of a TV series by the name of “Yanxi Palace,” in which the role of a deceased queen is called by netizens “the emperor’s white moonlight.”
Nǐ gē dōu bēnsì de rén le，zěnme hái bù jiéhūn？
Your elder brother is almost 40. Why is he still single?
Tā dàxué tán le yīcì liàn’ài，nàge nǚshēng chéng le tā niànniàn búwàng de báiyuèguāng。
He had a romance back in university, and that girl became his white moonlight (the cherished love of his life).