Moot can mean
These two seem to me to be opposites in many cases. I've seen people say "that's a moot point" in response to a proposal/idea: do they mean "that warrants debate" or "we should not bother talking about this"?
Is there a correct usage, or a way to use it while avoiding ambiguity? Or have I misunderstood the definition?
Good question. The American Heritage Dictionary in fact does advert to this duality in a separate Usage Note.
The adjective "moot" is originally a legal term going back to the 1500s. It derives from the noun "moot" in its sense of a hypothetical case argued as an exercise by law students. The noun "moot" in turn goes back to an Old English word meaning “a meeting, especially one convened for legislative or judicial purposes.” Consequently, a "moot question" is one that is arguable or open to debate. But in the mid-1800s, people also began to look at the hypothetical side of "moot" as its essential meaning, and they started to use the word to mean “of no significance or relevance.” Thus a "moot point", however debatable, is one that has no practical value. A number of critics have objected to this usage, but in our 2008 survey 83 percent of the Usage Panel accepted it in the sentence The nominee himself chastised the White House for failing to do more to support him, but his concerns became moot when a number of Republicans announced that they, too, would oppose the nomination. This represents a significant increase over the 59 percent that accepted the same sentence in 1988. Writers who use this word should be sure that the context makes clear which sense of "moot" is meant. It is often easier to use another word, such as "debatable" or "irrelevant".
[American Heritage Dictionary]
Please note that the bolding— wherever it appears— is mine.
There's a relevant article from the Columbia Journalism Review entitled Shhh! It doesn't matter — A "moot" discussion.
“Moot” is an old legal term. It originated in the twelfth century and meant either “A meeting, an assembly of people, esp. one for judicial or legislative purposes,” or the place the meeting was held, according to The Oxford English Dictionary. It evolved to mean an argument, then litigation.
In fact, in Britain, “moot” can still mean “arguable” or “debatable,” exactly the opposite of its American usage.
There's the OP's first definition: Subject to debate.
“Moot court” is so called because points are debated in them, not because the points debated are, well, pointless to debate.
There's the OP's second definition: Of no practical importance; irrelevant.
Somewhere around 1900, though, the fact that the “moot court” cases were hypothetical, or not worth arguing, began to overtake the traditional meaning of “moot” as arguable, according to Garner’s Modern American Usage. Some held out for the traditional meaning; as recently as 1980, John Bremner wrote of “moot” in Words on Words: “It is mistakenly used to signify that something is beyond argument, that there is no point in arguing the question, except in the technical legal sense that something moot is something previously decided.”
Moot court continues today, in which law students vigorously debate an issue which is of no practical importance (because the issue has already been decided in a real court and the moot court wouldn't have any effect on the existing ruling).
(While the purpose of the article was to correct the incorrect usage of mute point, the etymology and evolution of the word moot was still quite useful.)
