Language experts actually have theories about why people say they "could care less." Grammar Girl digs deep to find the root of the problem
What’s the Trouble?
People say they could care less when, logically, they mean they couldn’t care less.
The phrase "I couldn’t care less" originated in Britain and made its way to the United States in the 1950s. The phrase "I could care less" appeared in the US about a decade later.
In the early 1990s, the well-known Harvard professor and language writer Stephen Pinker argued that the way most people say could care less—the way they emphasize the words—implies they are being ironic or sarcastic.
Other linguists have argued that the type of sound at the end of "couldn’t" is naturally dropped by sloppy or slurring speakers.
Regardless of the reason people say they could care less, it is one of the more common language peeves because of its illogical nature. To say you could care less means you have a bit of caring left, which is not what the speakers seem to intend. The proper "couldn’t care less" is still the dominant form in print, but "could care less" has been steadily gaining ground since its appearance in the 1960s.
What Should You Do?
Stick with "couldn’t care less."
Here's an example from the TV show Psych:
Juliet O'Hara: Guess what today is.
Carlton Lassiter: It's not one of those touchy-feely holidays invented by card companies to goad me into buying a present for someone I couldn't care less about, is it?
- Maggie Lawson playing O’Hara and Timothy Omundson playing Lassiter