I'm reasonably sure that the phrase "I could care less" was introduced into American English by Jewish immigrants whose first language was Yiddish. Yiddish is mostly German, and German has a subjunctive mood which implies "I might do X, but I probably won't". "Could" is the nearest thing to subjunctive in English, so I think these immigrants (and their English speaking descendants) were reaching for something like "maybe I could care less, but probably not." Jewish-American comedians spread this use throughout the rest of the population.