Learn English – How long has the f-word been in use as an abusive term

Actually, the "reduce by 10%" meaning is not the classical sense, and is in fact a modern invention! So if at all decimate has been used in this sense, it's only in the modern period, not in any classical period. As the Merriam-Webster's dictionary of English Usage explains, decimate has had three main uses in English:

1. A specific Roman military practice of punishment (an army punishing its own soldiers), and only in this specific context (not a general-purpose "reduce by 10%"). The practice was that if a unit had exhibited cowardice or insubordination, one-tenth of the unit would be chosen at random, and clubbed to death by the other nine-tenths. You can read a five-page description of decimation in this book. Anyway, this sense carries over from Latin, and is attested in English since at least 1600.

2. A ten percent tax (esp. the one levied by Cromwell on the Royalists). This short-lived usage, attested since 1659, seems to have gone out of use (though the word tithe has taken some of its function).

3. The "modern" sense: emphatically destroy, devastate, severely reduce (not by just 10%) the numbers of, etc. This is in fact attested since 1663.

Now, it seems that Sir James Murray, primary editor of the first edition of the Oxford English Dictionary, inserted a definition of decimate as "to kill, destroy, or remove one in every ten of" (the "reduce by 10%" meaning) between sense 1 and sense 3, to have a "semantic bridge" between them. This definition was given without citations (unusual for the OED) — perhaps decimate had never been used in this sense in English till then.

And it hasn't been much used in that sense since, either.

[The only exception is in engineering, where "decimation" means reducing the number of samples (resulting in a lower 'resolution'), with no implication about the extent of the reduction: so you see phrases like "decimate by a factor of 4". In such a context, "decimate by a factor of 10/9" would effectively mean "reduce by 10%".]

"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.