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1.
The proper way to express how you don't give a crap about something - you really don't care about something and it's impossible to not care any more. The common but incorrect variant "could care less" (which would technically imply that you do care about something) is widely used because of either a translation mistake into English, or those who blindly use phrases without taking a moment to understand their actual meaning.
Person A: A company just got sued for patent infringement!
Person B: *yawn* Oh really? I couldn't care less.
by generic June 09, 2006
 
2.
A phrase meaning "The degree to which I care about this could not possibly go lower." Most people naturally interpret it to mean "I care about this as little as any human could; my degree of care is 0".

But it can also be interpreted to mean "I care about this to a certain extent (which may be a little or a whole lot) and there is no way in hell my caring is ever going to drop."

Arguments about this can get pretty heated, with Team Couldn't Care Less tending to care more.
"Guess what happened on Dancing with the Stars last night?"
"Please don't bother me with that, I really couldn't care less about it."

"Mr. Politician, do you care about America?"
"I'm glad you asked that. I care about America more than anything in the world, and I couldn't care less, not in a million years. You'll have to pry my care from my cold dead hands."
by Lenoxus July 05, 2012