This, from 'The Oxford Book of Word Origins' (entitled 'The Insect that Stole Butter')
I would recommend anybody who hasn't got a copy of this should get their hands on one.
It's probable one of the most entertaining books on our shelves, not to mention useful
Jim Carroll

""DECIMATE [LME] When Roman legions mutinied, they would be decimated—one in every ten men would be selected by lot and executed. In its first recorded use in English, in the late 16th century, decimate refers to this practice, but by the mid 17th century people were using it of other acts of killing, destroying, or removing one in ten. They then lost sight of the military context, and soon any severe loss or destruction could be described as decimation. See also DECADE""
("LME stands for Latin Middle English, by the way)