I know this is going to step on the toes of some here (many I count as friends). However, I am not thinking of anyone in particular when I write this, so please do not take this as a direct attack at your manner of speaking!!!
The phrase is: "I
couldn't care less" . . .not, "I could care less"!!!!
The reason it is a slam to tell someone that you "couldn't" care less is that, given all of life's possible scenarios about which to have concern, you could not care less about this particular one . . .it is the lowest concern on the totem pole of life for you; nearly at the point of non-existant caring!!! See?!?!?!?
However, if you say "I could care less," what you are actually saying is that you
COULD care less about the subject/person than you actually do, implying that you DO CARE about it/him/her to some degree!!!
I know, I know. this is a really stupid thing to have upset a person, especially with all of the really important stuff going on around us every day. I try to never correct anyone directly for this infraction, because it is so picky, but it ansolutely drives me INSANE!!!! So please, if you disagree, feel free to do so, but don't say you "could care less" about my pet peeve . . .I'm begging you!
P.S. I'm fairly certain the French started this practice of mis-stating a perfectly good slam!