"I couldn't care less."

It is an idiom. It means you don't care (and it is emphatic). Idioms don't have to make sense.

This one does make sense though. You can read it as.. I care so little that I couldn't possible care any less., The "less" is a quantifier on how much you are caring (I hope that helps). It is saying you are caring the absolute minimum amount possible.

You also hear Americans say the opposite "I could care less", which means the same thing even though it seems to be saying the opposite.

But, as I told you, idioms don't have to make sense.

If you took the opposite meaning, I could care less, it would need a bit more to make perfect sense. IE. I could care less if.....(something happened that would make me care less.)

I couldn't care less means that regardless of future events and circumstances there is nothing that would make me care any less about this than I do now.

Btw, people who use the term could care less are ignorant of the true phrase, like people who say escape goat, tenderhooks and pass mustard.

It's called an eggcorn.

To answer your other questions, "less" in this case is an adverb (modifying the verb to "care").

Could and can are simply two different verbs with different definitions. The phrase "I can't care less" makes grammatical sense. The auxiliary verb "can" relates more to one's ability to do something. The verb "could" relates more to possibility. (The meanings are similar).

Simplified - "I don't care".
Anything else is embelishment - Which allows (user), whether intended or not, to be categorised.

What's wrong with embellishment? Normal people use these idioms and if you are going to be fluent in English you have no know them.

"I could care less" is ignorant; it is not a legitimate idiom. Stupid people don't think about what they say; hearing this phrase gives us a clue that we shouldn't take their opinions seriously. It fits the pattern of "I could eat less," so it actually means "I care too much," which is the opposite of what they think they are saying.

By the way, the bimbo Valley Girl word "clueless" is also illogical, because the ignorant conformist way it is being used would fit the absurdity of saying, "When it comes to doing crossword puzzles, he is clueless."

Absolutely, not a single, thing, wrong with embelishment.
But 'truth'......
Is one word.

I agree.

"NORMAL PEOPLE"!?

What the flying f**K is that?

This is a thread started by an English language learner. How normal everyday people use the language is the point.

'Normal-everyday-people'?
Lost me there, max.
I deal with k's of peeps throughout my week - None of them are less than Nuts, nor am I.
What the ******* is Normal, please?

People who regularly use embellishments in their speech.

I'm guessing sad, middle aged, American white guys who spend all their time whining about how hard it is to be a man.

What about happy, middle aged, American White guys who spend their time whining about how hard it is to be a man?

You don't think we would do all this rape apologistic wife-beatin' misogyny if it didn't make us happy, do you Izzy?

Doing so intentionally.
Follow the plot - It's expected.