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.”
Curzan points to Michael Quinion’s explanation of this phrase from his website World Wide Words. “He tells us that it used to mean, and for many speakers still means that somebody seems nice, but underneath they’re catty or mean. So it seems like ‘butter wouldn’t melt in their mouth’ because they are so nice.
"He goes on to say that for some speakers, now, it just means that somebody is nice. I have to admit that I don’t use this expression, but what I thought it meant was that somebody was so cold, or so mean, that ‘butter wouldn’t melt in their mouth,’” Curzan says.
And then there’s this expression, “You’re a sight for sore eyes.” Many speakers over the age of 20 or 25 believe it’s a compliment.
“I poll students in my undergraduate classes and at least half of them say, ‘Oh no, it’s a bad thing to be sight for sore eyes.’ It’s as if you’re an eye sore, you create sore eyes. I’ve talked with 10-year-olds and 12-year-olds, and for them it seems to almost always be a bad thing,” Curzan says.