Here's a good thread on the issue, with Google Ngram evidence for the original phrase (we know which one that is ;), and another post with a hypothesis, echoed

Here's a good thread on the issue, with Google Ngram evidence for the original phrase (we know which one that is ;), and another post with a hypothesis, echoed in a couple of subsequent posts, for the emergence of the rephrasing: https://english.stackexchange.com/questions/283149/when-did-i-could-care-less-rather-than-i-couldnt-care-less-become-popular https://english.stackexchange.com/questions/283149/when-did-... That rephrasing hypothesis is the best I've seen yet, but still not especially persuasive.