I agree that couldn't makes more sense but it's an idiom so this is for you, specifically the bold part : p [quote]Etymologists suggest that “I could care less” emerged as a sarcastic variant employing Yiddish humor. They point to the different intonations used in saying “I couldn’t care less” versus “I could care less.” The latter mirrors the intonation of the sarcastic Yiddish-English phrase “I should be so lucky!” where the verb is stressed. The argument of logic falls apart when you consider the fact that both these phrases are idioms. [b]In English, along with other languages, idioms are not required to follow logic, and to point out the lack of logic in one idiom and not all idioms is…illogical.[/b] Take the expression “head over heels,” which makes far less sense than the expression “heels over head” when you think about the physics of a somersault. It turns out “heels over head” entered English around 1400, over 250 years before “head over heels,” however, the “logical” version of this idiom has not been in popular usage since the late Victorian era.[/quote]