A recent Doctor Who episode, “A Town Called Mercy,” featured a character who described a war that “decimated” more than half his planet. Something about that seemed mathematically problematic, so I thought I might investigate what “decimate” actually means. As it turns out, the Doctor Who character had it both right and wrong.

“Decimate” can mean “to reduce drastically especially in number” or “to cause great destruction or harm to.” That would be the Doctor Who meaning.

If you look at the word etymologically, though, you get the sense that the number 10 ought to be involved somehow. That’s where some older definitions come in: “to select by lot and kill every tenth man of” or “to exact a tax of 10 percent from.”

So using the original meaning, the character was super-mega-wrong. But no one cares anymore, so go nuts.