je ne sais quoi
A positive or pleasant quality that is difficult to define. This French phrase translates to "I don't know what" in English. She had a certain je ne sais quoi that men found quite attractive.
ˌje ne sais ˈquoi
/%Z@ n@ seI "kwA:/ (from French, often humorous) a good quality that is difficult to describe: He has that je ne sais quoi that distinguishes a professional from an amateur.It has a certain je ne sais quoi that really appeals to me.The meaning of the French phrase is ‘I do not know what’.