What is the X in X-mas?
early 15c., from Middle French recompense (13c.), related to recompenser "make good, recompense" from Late Latin recompensare (see recompense (v.)).
c.1400, "to redress," from Middle French recompenser (14c.) and directly from Medieval Latin recompensare "to reward, remunerate," from Latin re- "again" (see re-) + compensare "balance out," literally "weigh together" (see compensate). From early 15c. as "to compensate." Related: Recompensed; recompensing.