你妹 [nǐ mèi] n. Literally “[Fuck] your our sister” which is derived from “Fuck your mother/grandmother/uncle”—this term is a curse word.
Every time Duan Yu pursued a woman, his father would tell him: [She’s] Your sister!!
[Note: Duan Yu is a fictional character in Jin Yong’s novel Demi-Gods and Semi-Devils who unknowingly chases and falls in love with many of his half-sisters. Now “your sister” is also a popular Chinese curse/expletive used predominantly among young people.]