What is the X in X-mas?
"stone serving as a landmark," Old English mærstan "boundary stone," from mære "boundary, object indicating a boundary," from Proto-Germanic *mairjo- (cf. Middle Dutch mere "boundary mark, stake," Old Norse -mæri "boundary, border-land"), related to Latin murus "wall" (see mural). Hence also meresman "man appointed to find boundaries" (of a parish, etc.).