Here's a good thumb-sucker on how to know when to stop using a word's original definition when its meaning is changing through common usage:
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.
[. . .]
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.
He includes his "arbitrary metric" that uses Google search to determine whether a word has sufficiently tipped over into a new usage to drop the old one.
I must confess that, as someone who makes a living using words, I display the "hypercorrect linguistic behavior" from time time -- that is, I can be a real jerk about word usage. At least I'm not the kind of jerk who says to someone, "You know, I don't think that's quite the word you think it is." It's much cooler to use a word the "right" way, have someone correct you, then call him on his own ignorance.
("President Nixon brought a thitherto unheard of level of deceit to the White House."
"I say, old boy, don't you mean 'hitherto' ?"
"Oh, no, good sir, you are mistaken. " 'Hitherto' means 'until now.' If you want to say 'until then,' you need 'thitherto.' ")
I have to remind myself sometimes that a dictionary is not a bible dictating proper usage. It is merely a compilation of common usage -- how everybody says a word and what they mean by it are how it is said and what it means. It can be fun to be a word snob, but it's not really in the spirit of language as a living, dynamic thing.
