It's a Yiddishism, as far as anyone has been able to trace it. There is an implied second half, as if to say that caring less would involve some sort of effort

It's a Yiddishism, as far as anyone has been able to trace it. There is an implied second half, as if to say that caring less would involve some sort of effort or commitment beyond what is warranted. It comes to us the same way as already playing the part of the German/Yiddish schon does ("Will you hurry up already!"). Unfortunately, the 18th-century idea that English should play by the strict rules of logic on a structure that would make learning serious academic languages like Latin and Greek easier has become pervasive. The grammar textbooks of my youth no longer carried titles like An accidence to the English tongue, chiefly for the use of such boys and men as have never learned Latin perfectly, and for the benefit of the female sex, also for the Welch, Irish, Scotch and foreigners, but the sentiment was largely the same, and the grammar they described were every bit as foreign to the actual grammar of English as Latin itself.