I can't see how to define "accept" except as the opposite of rejection. You can understand what someone is trying to say, but then mock them for saying it "stupidly". You can also just refuse to repeat it. If you accept someone's usage, you probably do neither of these (you might make fun of someone for the heck of it, or not get a chance to use the new term, but you get the idea). Social dynamics, basically. And yes, this metric might produce different answers at different cultural strata.

If you understand what they're saying, you've already "accepted" the meaning/spelling/usage/etc. Turning around and mocking people or refusing to use the term is a separate issue that doesn't necessarily reflect well on that person.

We already have a word for that, though. It's "understand". And while I see what you mean about mockery, refusing to use a term/idiom is everyone's prerogative. I, and probably you as well, simply don't use certain widely accepted phrases because I think they're dumb.

Anyway, I came up with a better definition: accepting a word/usage is believing that it's a good way to express whatever it was.

It means you do not promulgate the usage. I understand perfectly well that some people who say "literally" actually mean "very," even though I do not accept this semantic change in "literally."

I myself will never use "literally" to mean "very," which differentially lowers the chance of this new meaning becoming standardized. Over time, either most people will do the same and the proposed novel usage of "literally" to mean "very" will die out, or else most people will collectively adopt the new meaning.

Empirically, almost all novel usages die out very quickly and are forgotten. Read some slang from the 1920s -- it's already almost incomprehensible to a modern reader.