Why was "tantrum" trending last week?
1610s, "unconcerned," the sense we now would ascribe to uninterested, with the sense of "impartial" going to disinteressed (c.1600). See dis- + interest. Modern sense of disinterested is first attested 1650s. As things now stand, disinterested means "free from personal bias," while uninterested means "caring nothing for the matter in question." Related: Disinterestedly; disinterestedness.