There is a famous French Music-Hall song, written and sung by Yvette Guilbert, called Madame Arthur. The lines at the ends of each stanza describe how Mme Arthur lives a high life by dint of her “je ne sais quoi.” It is used as a noun here and translates approximately as “her certain something.”
The song in its entirety lets you know, subtly, what that “je ne sais quoi” is. Here is Guilbert, singing it, gorgeously, in 1934. http://www.youtube.com/watch?v=0tUAMlXxN8U
Sample: Chacun applaudissait sa grâce
Et surtout son je ne sais quoi !
Toulouse-Lautrec did some famous posters of her.