The long-standing mystery as to how the idiomatic phrase in American English, “I could care less,” could mean the same thing as its negation, “I couldn’t care less,” is to be explained as an ELLIPSIS. The full version underlying it is: “I could care (even) less, if I cared to (care).”
MICHAEL SHAPIRO
By Thoth I think you’ve got it. How long have people been saying it? I remember saying it back in the mid-1970s.
I think It’s been around since the ’50s.