What is the X in X-mas?
mid-14c., from Old French bordure "seam, edge of a shield, border," from Frankish *bord or a similar Germanic source (cf. Old English bord "side;" see board (n.2)). The geopolitical sense first attested 1530s, in Scottish (replacing earlier march), from The Borders, name of the district adjoining the boundary between England and Scotland.
c.1400, "to put a border on;" 1640s as "to lie on the border of," from border (n.). Related: Bordered; bordering.