What is the X in X-mas?
"an inexpressible something," French, literally "I do not know what."
[T]hey are troubled with the je-ne-scay-quoy, that faign themselves sick out of niceness but know not where their own grief lies, or what ayls them. [Thomas Blount, "Glossographia," 1656]
That little something; a quality that eludes description: “The Mona Lisa's smile has a certain je ne sais quoi.” From French, meaning “I don't know what.”