1610s, "unconcerned" (the sense that now would go with uninterested), from dis- "opposite of" + interested. The sense of "impartial" originally was in disinteressed (c. 1600), from Old French desinteresse, and subsequently passed to uninterested. The modern sense of disinterested, "impartial, free from self-interest or personal bias, acting from unselfish motives," is attested by 1650s.
By late 18c. the words had sorted themselves out, and as things now stand, disinterested means "impartial," uninterested means "caring nothing for the matter in question," and disinteressed has fallen by the wayside. Related: Disinterestedly; disinterestedness.
Disinterested and uninterested are sometimes confounded in speech, though rarely in writing. A disinterested person takes part in or concerns himself about the affairs of others without regard to self interest, or to any personal benefit to be gained by his action; an uninterested one takes no interest in or is indifferent to the matter under consideration .... [Century Dictionary]