this is one that gets much play in U.S. discussion groups; here is what the alt.eng.usage FAQ has to say:
"could care less". The idiom "couldn't care less", meaning "doesn't care at all" (the meaning in full is "cares so little that he couldn't possibly care less"), originated in Britain around 1940. "Could care less", which is used with the same meaning, developed in the U.S. around 1960. We get disputes about whether the latter was originally a mis-hearing of the former; whether it was originally ironic; or whether it arose from uses where the negative element was separated from "could" ("None of these writers could care less . . .") Henry Churchyard believes that this sentence by Jane Austen may be pertinent: "You know nothing and you care less, as people say." (Mansfield Park (1815), Chapter 29) Meaning-saving elaborations have also been suggested; e.g., "As if I could care less!"; "I could care less, but I'd have to try"; "If I cared even one iota -- which I don't --, then I could care less."
