What is the X in X-mas?
mid-15c., from Middle French exubérant and directly from Latin exuberantem (nominative exuberans) "overabundance," present participle of exuberare "be abundant, grow luxuriously," from ex- "thoroughly" + uberare "be fruitful," related to uber "udder," from PIE root *eue-dh-r- (see udder). Related: Exuberantly; exuberate; exuberating.
exuberant ex·u·ber·ant (ĭg-zōō'bər-ənt)
Proliferating or growing excessively.