Stupid people often complain about this phrase, insisting that "I couldn't care less" should be used instead. This is because they fail to realize that "I could care less" is an appropriate and historically popular idiom that dates back to the 1960s.
The very same morons will probably tell you, "But if you could care less, that means you actually cared about it at least a little bit in the first place." That is because the sarcasm is lost on them. Should you hear this, just smile and enjoy knowing that you are speaking to someone of low intelligence.
Reggie: "I could care less, man."
Pete: "Haha, you mean 'I couldn't care less.'"
Reggie: "No I don't. You're retarded."
jim: i could care less. i hate video games.
john: wait, so you could care less.... that means that you care somewhat. you mean "i couldn't care less"?
jim: what? no. i don't give a shit about your dumb video game.
john: you're a fucking retard.
2. It is not possible for me to be less interested in the subject than I am at this moment.
When unsure as to which definition should be applied, it can help to judge the person's intelligence - morons usually mean #2.
This is turn means 'I care so little about this topic that my level of caring is at its utmost minimum'
When people say 'I could care less', what they are actually saying is:
'My level of caring is not yet at its minimum, and there is scope to care less about this topic.'
...Which is one of the most useless things you can tell someone.
B: "I couldn't care less"
A: "This person is disinterested, therefore I shall refrain from further comment about such matters"
A: "The paint is nearly dry! Oh wow what a momentous occasion!"
B: "I could care less"
A: "Erm...ok...thanks for letting me know?" O_o
"She looks **amazing**."
"I know. I love how **everything** she's wearing goes **perfectly** together."
(Someone talks about something nobody cares about.)
"**I could care less.**"
"Don't you mean, 'I couldn't care less.' ?"
"Sure. Hey, you know what? Your outfit is **fantastic**."