je ne sais quoi
noun [ S ] uk/ˌʒə nə seɪ ˈkwɑː/ us/ˌʒə nə seɪ ˈkwɑː/
难以描绘（或表达）的好品质;妙不可言的特性（法语的‘I don't know what’）
French for 'I don't know what': a pleasing quality that cannot be exactly named or described
Although he's not conventionally attractive, he has a certain je ne sais quoi that makes him popular with the ladies. 尽管从传统意义上讲他并不吸引人，但他身上有种说不清的气质让他很受女士们青睐。