Via Eleanor Houck on Facebook, this poster from Grammarly:
Grammarly is peeving obtusely here, affecting to misunderstand an idiom — could care less — that’s been around for at least 60 years and is now a commonplace. No modern speaker should fail to understand the intended meaning of the idiom.
ADS-L and Language Log have been on this case for years now. Back in ADS-L on 5/23/09, Ben Zimmer noted:
When I looked a few years ago, I found “couldn’t care less” in print from 1944 and “could care less” from 1955 [with examples]
Back in 2005, Mark Liberman posted an inventory of Language Log postings on could care less (up to that point), including the important “Negation by Association” posting explaining John Lawler’s account of how the idiom developed, using the parallel of French ne (negator) > ne pas (strengthened negation) > pas (negator), in which what was originally a concomitant of negation becomes the mark of negation. For could care less, the development is from couldn’t care less (a semantically transparent construction that became a formula for expressing strengthened negation), with n’t as the mark of negation, to could care less, in which less is now the mark of negation.
But whether you appreciate this account of the history, you should appreciate what people are doing with the idiom and not just insist that millions of people are getting the expression wrong.