Few phrases set off as many arguments with as little confusion. Say "I could care less" and someone will point out that, logically, you've just admitted you care. Then they'll carry on understanding you perfectly.

The dictionaries side with understanding. As checked in October 2026, Merriam-Webster defines could care less and couldn't care less on the same page, with one meaning: not at all concerned about or interested in something. Cambridge lists "I couldn't care less" as UK informal, with "I could care less" as the US alternative. Oxford Learner's gives "could care less" as North American English.

The negative form is older. Merriam-Webster finds couldn't care less in American magazines in 1929 and 1934. There's a popular story that GIs brought it home from Britain after the Second World War; the dictionary calls that speculation, since the phrase was already in print earlier. Could care less, as a fixed phrase, appears by 1954.

Why drop the not? Nobody knows for sure. Merriam-Webster lists the usual theories — sarcasm, lack of education, and the phrase simply sounding better without it — and declines to pick one. What it does say is that people who expect informal idioms to behave logically are in for a lifetime of hurt.

The practical verdict: both are real, both are informal, and both mean the same thing. If your audience is British, mixed, or likely to be picky, say couldn't care less. It's the form nobody will argue with.