Expression: Je ne sais quoi
Pronunciation: [zheu n(eu) say kwa]
Meaning: I don't know what, something inexplicable
Literal translation: I don't know what
Notes: The French expression je ne sais quoi literally means "I don't know what":
Je ne sais quoi faire avec lui.
I don't know what to do with him.
Il a acheté du pain, des croissants, un gâteau et je ne sais quoi encore.
He bought bread, croissants, a cake, and I don't know what else.
Je ne sais quoi can be used with the preposition de plus an adjective, to refer to something difficult to pin down:
Cette musique a je ne sais quoi d'obsédant.
This music has something haunting about it.
La maison a je ne sais quoi de bizarre.
There's something bizarre about the house.
Note that the adjective always remains in the default, masculine singular form, because it is agreeing with the neuter je ne sais quoi rather that whatever noun is being described.
The expression may be preceded by the indefinite article un and the four words might optionally connected with hyphens:
Il y avait un je ne sais quoi de perturbant dans ses yeux.
There was something unnerving in his eyes.
Elle a un je-ne-sais-quoi qui m'intrigue.
She has a certain something that intrigues me, There's something intriguing about her.
English's borrowing of this expression is limited to the meaning in the final example: "a certain something" or "something inexplicable." The only adjective that can accompany je ne sais quoi in English is "certain":
She has a certain "je ne sais quoi" that intrigues me.
His house is very nice, but it lacks a certain "je ne sais quoi."