You’ve gotten into a dispute with a merchant, who sold you what you think is defective merchandise. Because the merchant is a member of the Better Business Bureau, you agree to settle the dispute in front of a “disinterested” arbiter.

Wait a minute! You don’t want someone who is “disinterested” to hear your case; you want someone who is “interested.”

Actually, you want both. The arbiter should be “interested” in hearing both sides, but should have no investment in either side; the arbiter should be “disinterested.”

“Disinterested,” in careful usage, means “objective” or “impartial,” someone who has no dog in the fight, like a judge. But, as Patricia T. O’Conner and Stewart Kellerman point out in their informative and entertaining new book of word myths, Origin of the Specious, “disinterested” actually started life in the seventeenth century as a synonym for “uninterested.” It added the “impartial” meaning soon after, and pretty much kept both meanings until the end of the nineteenth century. At that point, “strictarians” lost interest in having “disinterested” mean “uninterested,” and started insisting on the “impartial” meaning.

As we’ve heard many times before, that didn’t stop people from using “disinterested” as a synonym for “bored,” “uninterested,” or “indifferent.” Even usage guides that hammered on the difference started to cave in, though Webster’s New World College Dictionary notes that the use of “disinterested” to mean “uninterested” or “indifferent” is “a revival of an obsolete meaning” that “is objected to by some.”

There will be times, as in dealing with court cases or similar kinds of proceedings, when “disinterested” provides a nuance to emphasize the lack of “interest” (not attention). But, as O’Conner and Kellerman say, “it’s better to be understood than to be correct, especially when intelligent people can’t agree on what is correct.” So when in doubt, use more precise words: “objective,” “impartial,” “not interested,” “uninterested,” or even “I don’t care.”

Of course, if the arbiter should fall asleep as you’re presenting your case, that arbiter is both “disinterested” and “uninterested.” Let’s hope you don’t get one of those.

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Merrill Perlman managed copy desks across the newsroom at The New York Times, where she worked for 25 years. Follow her on Twitter at @meperl.