What is the X in X-mas?
early 14c., from Old French quantite, cantite (12c., Modern French quantité) and directly from Latin quantitatem (nominative quantitas) "relative greatness or extent," coined as a loan-translation of Greek posotes (from posos "how great? how much?") from Latin quantus "of what size? how much? how great? what amount?," correlative pronomial adjective, related to qui "who" (see who).
Latin quantitatem also is the source of Italian quantita, Spanish cantidad, Danish and Swedish kvantitet, German quantitat.