What is the X in X-mas?
early 14c., bikere, "to skirmish, fight," perhaps from Middle Dutch bicken "to slash, stab, attack," + -er, Middle English frequentative suffix. Meaning "to quarrel" is from mid-15c. Related: Bickered; bickering.
c.1300, skirmish, battle; from the same source as bicker (v.). In modern use, often to describe the sound of a flight of an arrow or other repeated, loud, rapid sounds, in which sense it is perhaps at least partly echoic.