What is the X in X-mas?
"an emperor," Old English casere, fallen from use after Middle English, but revived 1858 in reference to the German emperors of Austria and, after 1870, Germany, from German Kaiser, from Bavarian and Austrian spelling of Middle High German keisar, from Old High German keisar "emperor," an early borrowing of Latin cognomen Caesar. The Germanic and Slavic peoples seem to have called all Roman emperors "caesar" (cf. Old English casere, Old Norse keisari). Said to be the earliest Latin loan word in Germanic.