Arnold Zwicky thinks the peeve about the meaning of decimate should get a prize for its persistence in the face of usage facts: The Peevy for Lifetime Achievement.
We could debate the merits of the hopefully peeve as a rival candidate -- its heyday was much shorter, but the argument was often hotter, given that hopefully was a more common "transgression." But there's no debate about the prizeworthiness of the decimate question. Here's an excerpt from the column I wrote about it in the Boston Globe (July 1, 2007), after a reader insisted that "To decimate means to reduce by 10 percent, as was done by the Roman legions."
So it does, when you're speaking of the Roman Army, or of others who copied their harsh punishment for mutinous legions. But as an English word, decimate has always had a wider scope. Since the mid-17th century, says the Oxford English Dictionary, the word has also been used to mean "destroy or remove a large proportion of."
Nobody objected, it seems, for more than two centuries; there was the military decimate and the loose, emphatic decimate, each in its proper place. But in 1870, according to Merriam-Webster's Concise Dictionary of English Usage, the popular American language commentator Richard Grant White declared war on the degraded decimate.
White inspired some followers, but the purists met resistance from the start. In an 1885 essay, writer Grant Allen rebuked the "superfine English" crowd, saying there was "surely nothing very wrong or out-of-the-way" in expanding the sense of decimate.
H.W. Fowler, that notable stickler, also condoned the nonclassical usage, writing in Modern English Usage (1926) that it was "natural" to use decimate loosely. Current dictionaries and usage guides agree; decimate no longer means "reduce by 10 percent" -- if it ever did -- except in historical references. ...
If you pine for the classical decimate, though, you have a champion in [the late] language columnist William Safire. When he first addressed decimate, 25 years ago, he agreed with Fowler: "To limit the word's meaning to 'one-tenth' would be like limiting myriad to its literal '10,000.'" But a few years later, he quietly flip-flopped, warning readers that "unless purists persist, decimate will come to mean 'destroy a large part of.'" In 2004 he reaffirmed his faith: "Decimated means reduced by 10 percent."
But he was right the first time. If etymology governed usage, as he noted, we'd have to stop using myriad for "lots" -- and journal for anything not published daily, and honeymoon for wedding trips shorter than a month. That way lies lunacy.
Besides, we don't especially need a term that means "kill one in 10." As Barbara Wallraff notes in her book "Word Court," you're free to use decimate only in the narrow sense -- but "in that case, you won't be using the word very often."