Why do some people say, “I could care less” to mean they don’t care? It doesn't make sense. The expression is, "I couldn't care less," right?
“What has happened here, as far as I can tell, is that speakers are no longer parsing this phrase for every word. And this is what happens with idioms. Idioms take on a meaning that surpasses their parts,” says Anne Curzan, a professor of English at the University of Michigan.
“I think the ‘less’ there feels negative to speakers. It already says, ‘I don’t care,’ so for them, ‘I could care less -- I couldn’t care less,’ they mean the same thing,” she says.
Michigan Radio’s Rina Miller asks Curzan to explain this idiom, “Butter would not melt in her mouth.”