What is the X in X-mas?
early 13c., from Old French desirrer (12c.) "wish, desire, long for," from Latin desiderare "long for, wish for; demand, expect," original sense perhaps "await what the stars will bring," from the phrase de sidere "from the stars," from sidus (genitive sideris) "heavenly body, star, constellation" (but see consider). Related: Desired; desiring.
c.1300, from Old French desir, from desirer (see desire (v.)); sense of "lust" is first recorded mid-14c.