French: "I do not know what".
As far as english speaking people are concerned, this is spoken by snooty
types as to sound more sophisticated than they are, used humorously as such by people who *do* know what it means, and spoken by French when they need to say "I don't know what" in French.
Generally speaking, can be used if you're feeling particularly suave
to give an explanation of something of a certain unknown or indescribable quality.
See also unobtanium
"This house lacks what the French may call a certain je ne sais quoi," said the pretentious rich bastard.