"I could care less" = this means you do care; at least a little. If you want it to mean "I don't care" you need to say "I couldn't care less".

英語圏の若者を中心に「そんなことに興味ない」という意味で "I could care less" という表現を用いている人たちがいますが、正式には
"I couldn't care less"となります。