What is the X in X-mas?
late 14c., from Old French escarmouche "skirmish," from Italian scaramuccia, earlier schermugio, probably from a Germanic source (cf. Old High German skirmen "to protect, defend"), with a diminutive or depreciatory suffix, from Proto-Germanic *skerm-, from PIE *(s)ker- (1) "to cut" (see shear).
Influenced in Middle English by a separate verb skirmysshen "to brandish a weapon," from Old French eskirmiss-, stem of eskirmir "to fence," from Frankish *skirmjan, from the same Germanic source. Cf. also scrimmage. Other modern Germanic forms have an additional diminutive affix: German scharmützel, Dutch schermutseling, Danish skjærmydsel. Skirmish-line attested by 1864.
c.1200, from Old French escarmouchier, from Italian scaramucciare (see skirmish (n.)). Related: Skirmished; skirmishing.