Oversimplified, making you a victim of the "Ethymological fallacy"(*). Here are some professionals discussing the matter, at Language Log: [link|http://itre.cis.upenn.edu/~myl/languagelog/archives/000368.html|Post 000368], [link|http://itre.cis.upenn.edu/~myl/languagelog/archives/001180.html|Post 001180], [link|http://itre.cis.upenn.edu/~myl/languagelog/archives/001182.html|Post 001182], [link|http://itre.cis.upenn.edu/~myl/languagelog/archives/001202.html|Post 001202], [link|http://itre.cis.upenn.edu/~myl/languagelog/archives/001207.html|Post 001207], [link|http://itre.cis.upenn.edu/~myl/languagelog/archives/001209.html|Post 001209], [link|http://itre.cis.upenn.edu/~myl/languagelog/archives/001256.html|Post 001256], [link|http://itre.cis.upenn.edu/~myl/languagelog/archives/002063.html|Post 002063], [link|http://itre.cis.upenn.edu/~myl/languagelog/archives/002253.html|Post 002253](+), and here are some of their sources: A part of [link|http://www.yaelf.com/aueFAQ/mifcouldcareless.shtml|the alt.usage.english FAQ], and [link|http://www-personal.umich.edu/%7Ejlawler/aue/giveadamn.html|John Lawler's "Give a damn" piece].

HTH!



(*): Google for it, preferably at LL.
(+): I've tried to collect an exhaustive but not exhausting -- and relevant! -- sample from the somewhat larger number of posts that in some fashion touched on the subject (i.e, contained the phrase "could care less").