I could care less
You probably mean “I couldn’t care less,” which means that you flat out don’t care at all. You care about whatever that thing is—say, grammatical errors—in the least possible amount. If you could care less, then you do care about grammatical errors a little bit, and it would technically be possible for you to care even less than you do now.
Check out these seven words you never realized were examples of onomatopoeia.