What's That Je ne Sais Quoi, That Indefinable Thing She Has?

This expression is everywhere in English, even dictionaries

Je Ne Sais Quoi - French Expression
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"Je ne sais quoi" is a French idiomatic expression used so much in English that it has made it into the leading English dictionaries. In other words, it has been assimilated into the English language.

Merriam-Webster describes je ne sais quoi as "something (such as an appealing quality) that cannot be adequately described or expressed," as in "This woman has a certain je ne sais quoi that I really like." In French, Larousse calls je ne sais quoi "a thing one wouldn't know how to define but whose existence is understood intuitively."

'Je ne sais quoi' in French

In French, the expression je ne sais quoi literally means "I don't know what." It is often used for its literal meaning, not as an idiom. For example:

J'ai fait la vaisselle, le ménage, le répassage, et je ne sais quoi (d'autre) encore. > I did the dishes, the house-cleaning, I ironed, and I don't know what else.

How the French Use It

But the French also use it as we do in English: a quality you cannot describe. We connect je ne sais quoi to the adjective describing it with de, like this:

Cette fille a je ne sais quoi de fascinant. > "There is something fascinating about that girl."

Note that the adjective is always masculine singular, even if the sentence refers to a girl or a feminine noun. The adjective should agree with je ne sais quoi, which is masculine, singular.

Two Spellings in French

Or we can also use it, as in English, as a noun: un je ne sais quoi or hyphenated as un je-ne-sais-quoi.

Both spellings are correct. And we often use it with certain, as in English:

Elle avait un certain je-ne-sais-quoi de spécial : l'expression de son regard peut-être. > She had a certain special je ne sais quoi—the expression in her eyes perhaps.

Finally, in spoken modern French, the je and the ne glide together, making the expression sound like "jeun say kwa."

A Word About Spelling

This is a common expression that is recognizable in its correct spelling of je ne sais quoi. It is even in English-language dictionaries. So there really is no excuse for misspelling this classic phrase as "jena se qua," etc., as some anglophones tend to do. Just look it up in the dictionary. That woman with the special something will thank you.

Additional Resources

French expressions used in English 
quoi in French
irregular conjugation of the verb savoir