What is the X in X-mas?
c.1300, from Old French convertir, from Vulgar Latin *convertire, from Latin convertere "turn around, transform," from com- "together" (see com-) + vertere "to turn" (see versus). Originally in the religious sense. The Latin word is glossed in Old English by gecyrren, from cierran "to turn, return." Related: Converted; converting.
1560s, from convert (v.). Earlier was convers (early 14c.).