Why is the ninth month called September?
mid-15c., from Late Latin decimationem (nominative decimatio), noun of action from past participle stem of Latin decimare "the removal or destruction of one-tenth," from decem "ten" (see ten). Earliest sense in English was of a tithe; punishment sense is from 1580s; transferred sense of "much destruction, severe loss" recorded from 1680s.
c.1600, in reference to the practice of punishing mutinous military units by capital execution of one in every 10, by lot; from Latin decimatus, past participle of decimare (see decimation). Killing one in ten, chosen by lots, from a rebellious city or a mutinous army was a common punishment in classical times. The word has been used (incorrectly, to the irritation of pedants) since 1660s for "destroy a large portion of." Related: Decimated; decimating.