When I was in the 7th grade I heard this coloquilism used for the first time, when my Civics/PE coach used that phrase all the time.
My gut reaction was that it wasn't proper English. At least imo it isn't polite...kinda belittling in a way. (Ref Octavius' post: i.e. someone includes the potential for caring less in the wording, and presumes to to do forewith.) That was an excellent summation of "I could care less" Octavius.
So, it's kinda an arrogant or authoritarian way of saying "I couldn't care less." Definitely rude, crude and never used in polite society. It expresses disdain.
Les