What is the X in X-mas?
1590s, from French facétieux, from facétie "a joke" (15c.), from Latin facetia "jest, witticism," from facetus "witty, elegant, fine, courteous," of unknown origin, perhaps related to facis "torch."
It implies a desire to be amusing, often intrusive or ill-timed. Related: Facetiously; facetiousness. "Facetiæ in booksellers' catalogues, is, like curious, a euphemism for erotica." [Fowler]