je ne sais quoi (n.)youdaoicibaDictYouDict[je ne sais quoi 词源字典]
"an inexpressible something," French, literally "I do not know what."
[T]hey are troubled with the je-ne-scay-quoy, that faign themselves sick out of niceness but know not where their own grief lies, or what ayls them. [Thomas Blount, "Glossographia," 1656]
[je ne sais quoi etymology, je ne sais quoi origin, 英语词源]