English is more than happy to pick up loan words and phrases from other languages and use them with abandon. "Je ne sais quoi" is one of them. It literally means "I don't know what"; it figuratively means "cool - but in a classy way. Also, I have big vocabulary."

In many books and movies, English speakers who use French phrases like that are usually trying to seem smarter and better educated than other people. The most common usage for that phrase is something like "[It] has a certain je ne sais quois" which is roughly understood to mean that there is something about [it] that is difficult to put into words. It could be 褒め or it could be 皮肉, but there is no real meaning to it.

This is a not very often used French phrase that gets used in movies and by English students that are learning French because it sounds cool.

wow thanks for your comment on this old question!

there are 2 possible use : "je ne sais quoi [te dire/te repondre/...]" : just means I don't know what to [say/answer/...] but it's more likely you find this in english in the form of "un je ne sais quoi" : corresponds to what Christine describes : something difficult to describe with words. So it's something used when you try to describe something difficult to describe, generally based on feelings, emotions, ...