I once laughed when a child with chicken pox pointed out to me a single spot, calling it a "pock". I had been under the impression that they had mistakenly thought the word "pox" was plural, and so could be singularised.
Turns out I was wrong. "Pock" is a perfectly valid word.
That got me thinking about which word came first. Was the word "pock" invented, based on the existing word "pox"? Or did "pock" always exist, and "pox" is merely the shortened version of the plural "pocks"?
"Pock" is the older word. "Pox" is the variant of this word. According to etymonline.com:
Pock: O.E. pocc "pustule," from P.Gmc. *puh(h)- "to swell up, blow up" (cf. Du. pok, Low Ger. poche), from PIE base *bhu- "to swell, to blow." The plural form, M.E. pokkes, is the source of pox, which since early 14c. has been used in the sense "disease characterized by pocks.
Pox : late 15c., spelling alteration of pockes, pl. of pocke
As you can see, "pock" originated earlier, by about 100 years, and then there was a sudden shift to spell it 'x' instead of 'cks', due to unknown reasons to me, but "pock" is definitely older.
