What does the French term ‘Je ne sais quoi’ mean?

Read the given passage and answer the questions that follow.

“It is not a lip or eye we beauty call/ But the joint force and full result of all,” wrote Alexander Pope, the English intellectual poet and contemporary of John Dryden. The very idea of beauty is subjective because we never take it in totality.
Years ago, one saw a painting at an art museum in Warsaw, Poland. Some unknown Polish painter made it during World War I. It was about an undraped woman, who had a chiselled body but no face. There was a comment in the corner of that painting: Impose your idea of beauty on it and think of a face for her.
Everyone will visualise according to his one idea of beauty. This is indeed intriguing. Beauty, especially feminine beauty, is always stereotyped and reduced to concrete and unaltered idioms. The core concept of beauty lies in being abstract. ‘Koi soorat de do ke tarasta hoon main/ Meri taameer ki mitti abhi narm hai dekho’ – Give me a face, an appearance; my clay’s still wet. The moment we try to concretise and quantify beauty, it loses its aura and its appeal. There is a term in French – ‘Je ne sais quoi’, I know not what. Beauty ought to have this quality that cannot be adequately described or expressed, yet that makes somebody or something more attractive and interesting.
Beauty is always an open-ended idea. Neither can you bind it, nor can you define it. It is like love, indefinable, yet so desirable. And like love, it can only be felt and experienced. You can never appreciate it superficially. Then it becomes voyeuristic and too quotidian.