Idiom: someone couldn’t care less
Many people use "could care less" but this doesn't make sense. If you could care less it would mean that you have some amount of caring left to give--that's not what this expression is intended.
However, this form is commonly used so don't be surprised if you hear it, especially in spoken English.
— I couldn't care less if all of my friends smoke. I’ll never do it because it’s bad for my health.
— The salesperson is so slow it's obvious she couldn't care less if we buy anything or not.
— My wife wanted to know if we should paint our house gray or blue but I honestly couldn't care less.
— My parents couldn't care less what I do on the weekends as long as I get good grades and stay out of trouble.
— We couldn't care less about your video game competition. Turn off your computer and go to sleep.
— I couldn't care less what we have for dinner as long as we get something to eat soon. I'm so hungry I'm about to pass out.
— It's so annoying that you couldn't care less about anything—I'm so tired of having to make all the plans and decisions in this relationship.
— First you said you couldn't care less where we eat and now that I've made a reservation you decide you hate Thai food.
— When I grew up we couldn't care less about sun protection so I never used sunscreen.
— Many young people feel despondent feeling people couldn't care less about them.
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