couldn't care less - Phrases/Idioms
Also, could care less. Be completely indifferent. For example, Pick whatever dessert you want; I couldn't care less, or I could care less about the editor's opinion. This expression originated about 1940 in Britain and for a time invariably used couldn't. About 1960 could was occasionally substituted, and today both versions are used with approximately equal frequency, despite their being antonyms.