Grammar Fiends

Mark and I stopped in the middle of a conversation the other night...completely stopped what we were doing and dug out the dictionaries and thesarusus (actually, dug out the laptop and searched Dictionary.com), to get to the bottom of our investigation...
Why do we say "One of a Kind" when we mean "special" or "unique"? This was Mark's question.
To say that someone or something is "one of a kind" is to say, literally, that that individual is "one of a set of many." But, of course, the idiom is meant to say the exact opposite, that is, that the individual is "unique."

Now, when I get a challenge like this (which comes around, oh, say, every hour or so in our household) I just can't stop till I beat it. I argued that according to the definition of idiom (according to Dictionary.com)--

"A speech form or an expression of a given language that is peculiar to itself grammatically or cannot be understood from the individual meanings of its elements, as in keep tabs on."--

it is the very fact that it doesn't make grammatical sense that makes it an idiom and gives it an accepted place in our modern language.

Mark did not buy this. Yes, but not all idioms say the exact opposite of what they mean.

I began looking up common idioms. His point was quite true. "I'm all ears"--while this idiom does not make literal sense (surely I do not mean to say that I have become a horrific mass of conglomerated ears)--it is also not the opposite of what it means--i.e. "I'm listening."

So what, then, drove someone at some time (probably a lot of someones since it stuck), to bring into existence this strange paradoxical saying: One of Kind?

Any thoughts?

Another paradoxical saying that once fell under discussion is: "I could care less"--used to mean, "I don't care at all." Our best guess is that people are saying it wrong. It should be "I couldn't care less," as in, I care so little that I couldn't be caring less if I tried.

--Grammar Fiend A