(4). In negative and conditional construction: a. not to care passes from the notion of ‘not to trouble oneself’, to those of ‘not to mind, not to regard or pay any deference or attention, to pay no respect, be indifferent’. ... (c) Colloq. phr. (I, etc.) couldn't care less: (I am, etc.) completely uninterested, utterly indifferent; freq. as phr. used attrib. Hence couldn't-care-less-ness. (d) U.S. colloq. phr. (I, etc.) could care less = sense (c) above, with omission of negative.