Infallibility

I'm an English teacher. I admit it freely. It's a better occupation than being, say, a drug dealer, but not nearly as much fun.
The problem is that people tend to think that I am the final authority on all things having to do with grammar and language. I get friends who phone me for advice. Sometimes I have to tell them, "It really doesn't make much difference," or "We really don't know," or "since language changes, that's in flux." As an example of the last -- someone asked me recently about the differences between uninterested and disinterested. I replied that traditionally, uninterested meant "having no interest in," and disinterested meant "having no opinion on." I continued that disinterested is acquiring the meaning that was formerly given to uninterested. That is, the sentence, "Charlie was disinterested in the outcome," could mean "Charlie had no opinion about the outcome," or "Charlie didn't care about the outcome."
This elicited puzzled silence. How could it be, my caller asked, that things could change like that? Shouldn't language stay put? Isn't there a proper English somewhere that we can adhere to?
Well, possibly, but "proper" changes. At one time, mob wasn't proper. If I remember right, mob is short for a Latin phrase mobus vulgaris -- "the common people in motion."
We can wish all we want, but the fact is language is changing even as we talk about it.