(I) could(n't) care less. Inf. It doesn't matter to me. (The less bears the heaviest stress in both versions. Despite the apparent contradiction, either reading of this—both the affirmative and negative—usually have the same meaning... see:

http://idioms.thefreedictionary.com/could+care+less

Could care less has been in use since 1955, the way I use it, see:

http://www.boston.com/bostonglobe/ideas/articles/2010/10/24/...

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