What is the X in X-mas?
late 15c., from adverb (late 14c.), from Old French perpendiculer, from Latin perpendicularis "vertical, as a plumb line," from perpendiculum "plumb line," from perpendere "balance carefully," from per- "thoroughly" (see per) + pendere "to weigh, to hang" (see pendant). As a noun from 1570s. Related: Perpendicularly; perpendicularity.