It is a common saying, but the saying is "couldn't care less"
They are commonly used synonymously. English as she is spoke is not a logical language.
https://www.merriam-webster.com/grammar/could-couldnt-care-l...
https://dictionary.cambridge.org/dictionary/english/could-ca...
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They are commonly used synonymously. English as she is spoke is not a logical language.
https://www.merriam-webster.com/grammar/could-couldnt-care-l...
https://dictionary.cambridge.org/dictionary/english/could-ca...