What is the X in X-mas?
Old English pocc "pustule, blister, ulcer," from Proto-Germanic *puh(h)- "to swell up, blow up" (cf. Middle Dutch pocke, Dutch pok, East Frisian pok, Low German poche, dialectal German Pfoche), from PIE root *beu- "to swell, to blow" (see bull (n.2)). Middle French pocque is from Germanic. The plural form, Middle English pokkes, is the source of pox, which since early 14c. has been used in the sense "disease characterized by pocks."
"to disfigure with pits or pocks," 1841. Related: Pocked; pocking.
The characteristic pustular cutaneous lesion of smallpox.