"I couldn't care less" vs. "I could care less"

I couldn't care less is another way to say "I don't care at all."

It uses a rhetorical device called litotes, in which you negate the opposite of what you mean. When you say that your older brother is not the most considerate guy, you're using litotes to suggest that he's quite inconsiderate. Likewise, if you couldn't care less, your level of concern has hit absolute rock bottom — there's nowhere lower for it to go, as in the following examples:

Even with their big, fancy house on the corner, those Darcy boys couldn't care less about what';s happening on this block, much less this neighborhood. (Pride)

Even District residents who couldn't care less about sports will feel the short-term and long-term effects of Major League Baseball's trip to town. (Washington Times)

The confusion between couldn't care less and could care less arises from the differences between spoken and written language. If you say "I could care less" in a sarcastic tone, then it can have the same effect as couldn't care less: the sarcasm serves the same function as the litotes, suggesting that you mean the opposite of what you're saying.

But sarcasm doesn't always show up on the page, so you should always stick to the correct expression: couldn't care less. The sentences below don't make sense, because without the negating word not, these sentences literally state that their subjects could care less — and if their level of concern could drop to a lower level, then they must care at least a little:

And he could care less if you like him or hate him. (Fox News)

The fact is, salmon could care less about arbitrary political boundaries. (Seattle Times)

Remember, unless you're speaking in a voice dripping with sarcasm, always use couldn't care less to express how little you care.