Mark Israel
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: "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."
Recently encountered has been "could give a damn", used in the
sense "couldn't give a damn".
An earlier transition in which "not" was dropped was the one that
gave us "but" in the sense of "only". "I will not say but one
word", where "but" meant "(anything) except", became "I will say but
one word."
Other idioms that say the opposite of what they mean include:
"head over heels" (which could mean turning cartwheels, i.e. "head
over heels over head over heels", but is also used to mean "upside-
down", i.e. "heels over head"); "Don't sneeze more than you can
help" (meaning "more than you cannot help"; "help" here means
"prevent"); "It's hard to open, much less acknowledge, the letters"
(where "less" means "harder", i.e. "more"); "I shouldn't wonder if
it didn't rain"; "I miss not seeing you"; and "I turned my life
around 360 degrees" -- not to mention undisputedly ironic phrases
such as "fat chance", "Thanks a *lot*", and "I should worry".
http://alt-usage-english.org/excerpts/fxcouldc.html
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