Suppose a friend said to you, "I know you're disinterested, so I want to ask you a question presently." Then he didn't say anything. Would you be momentarily nonplussed?
Quite likely, yes. The above paragraph contains four words whose primary definitions have changed or are currently changing. Disinterested traditionally meant "impartial," and now is generally used to mean "uninterested." Presently has gone from "shortly" to "currently"; momentarily from "for a moment" to "in a moment"; and nonplussed from perplexed to unimpressed, or fazed to unfazed. To lend support to my theory that the new meanings now dominate popular usage, I gave an ungraded and anonymous quiz to one of my college classes—an advanced writing seminar. Here is the percentage who gave the "wrong"/new definition:
We all know that words change their meanings all the time, sometimes glacially (the prescriptivists have long been fighting on behalf of the "impartial" sense of disinterested) sometimes relatively quickly (that nonplussed thing snuck up on me). * But this fact raises a question (it doesn't beg the question—that means something else): How long should we hold on to a word's old meaning?
This is a subset of the larger issue—an ethical one, really—of how we should deploy our language knowledge. Some people—often children of English teachers or Anglophiles—proudly wear their knowledge on their sleeve, and adopt hypercorrect linguistic behavior. Take Ray Magliozzi, the less laughter-prone of NPR's Car Talk guys, who turns his sentences into pretzels so as to avoid ending them with prepositions: a "rule" that has been out of favor for roughly half a century. (Ray consequently favors the phrase "with which.") I actually heard him use the word "shall" on last week's show. A subclass of this group favors ur-renditions of common expressions. Adopting the diction of George Gissing or Walter Pater, they will choose stamping (instead of stomping) grounds, champing (instead of chomping) at the bit, pompons (instead of pompoms), or titbits (instead of tidbits). * Such archaism seems designed to attract attention, and nothing more.
But using a meaning on its way to extinction can be nobler than such exhibitionism. Balancing the possibility that you'll confuse your audience, and the prospect of appearing pretentious or dorky, is the chance that the old meaning could be a really good meaning, which no other word conveys precisely. There is no exact synonym for (the old-fashioned) disinterested, for example. In such cases, keeping a "legacy" sense in circulation is laudable activism in pursuit of semantic sustainability—as if you found some members of a near-extinct species of mollusk and built a welcoming environment in which they could breed.
So, pedantry on one side, conservation on the other. What's needed is an algorithm to help you decide where on the continuum a particular word or expression lies.
Guess what: I have such an algorithm! Or, more precisely, I have a somewhat arbitrary metric. In the chart below, the number under the percent sign indicates the proportion of the first 20 hits on a Google News search reflecting a word's oldmeaning. The "utility rating" is my view of how valuable that old meaning is. Lion's share gets a 0 because it is a cliché and one can express the traditional meaning simply by saying all. Fortuitous gets a 2 because accidental or coincidental mean pretty much, though not exactly, the same thing. Disinterested gets a 3.
To get the score, I took the percentage of Google hits that used the old meanings and added the utility ratings: 20 points for 1, 40 for 2, and 60 for 3. The final score correlates roughly with academic grades. That is, 65 or better means the old sense still passes, and you should feel free to use it. If it doesn't pass, you can either convert to the new sense or, if that's too painful, avoid the word entirely.
If you are champing at the bit to decimate me, be ready, because I will be done presently.