What is the X in X-mas?
early 13c., from Old French devocion "devotion, piety," from Latin devotionem (nominative devotio), noun of action from past participle stem of devovere "dedicate by a vow, sacrifice oneself, promise solemnly," from de- "down, away" (see de-) + vovere "to vow," from votum "vow" (see vow).
In ancient Latin, "act of consecrating by a vow," also "loyalty, fealty, allegiance;" in Church Latin, "devotion to God, piety." This was the original sense in English; the etymological sense, including secular situations, returned 16c. via Italian and French.