From Wiktionary: "The definition reduce by one in ten is occasionally cited as "the correct" definition, with severely reduce considered a misconception, arrived at by reading decimate as to reduce to one-tenth rather than by one-tenth."
"The Cambridge Guide to English Usage states that the nonspecific use of this word to mean devastate or severely reduce the numbers of is "nowadays the commonest use of the word in both British and American English, and it’s registered without comment in modern dictionaries."
A cohort (roughly 480 soldiers) selected for punishment by decimation was divided into groups of ten; each group drew lots (sortition), and the soldier on whom the lot fell was executed by his nine comrades, often by stoning or clubbing. The remaining soldiers were often given rations of barley instead of wheat (the latter being the standard soldier's diet) for a few days, and required to camp outside the fortified security of the marching camp.Because the punishment fell by lot, all soldiers in the group were eligible for execution, regardless of the individual degree of fault, or rank and distinction, unless rigged to eliminate the mutiny ringleaders. The leadership was usually executed independently of the one in ten deaths of the rank and file.