Corruption of the adjectivedisinterest/disinteressed. The sense of lacking interest is likely fromdis- +‎interested.

• IPA^(key):/dɪˈsɪnt(ə)ɹɛstɪd/

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disinterested (comparativemoredisinterested,superlativemostdisinterested)

1. Having nostake orinterest in theoutcome, and noconflicts of interest; free ofbias,impartial.[from 17th c.]

   Synonyms:detached,imprejudicate,unbiased;see alsoThesaurus:impartial

   Antonyms:biased,interested,partisan,prejudiced

   • 1791,James Boswell,Life of Samuel Johnson:

     Sir, you have a right to that kind of respect, and are arguing for yourself. I am supporting the principle, and amdisinterested in doing it, as I have no such right.

   • 1919,W[illiam] Somerset Maugham, “chapter 1”, inThe Moon and Sixpence,[New York, N.Y.]:Grosset & Dunlap Publishers […],→OCLC:

     With hisdisinterested passion for art, he had a real desire to call the attention of the wise to a talent which was in the highest degree original; [...]

   • 2011,Steven Pinker,The Better Angels of Our Nature, Penguin, published2012, page220:

     People are better off abjuring violence, if everyone else agrees to do so, and vesting authority in adisinterested third party.

   • 2014 April 12, Michael Inwood, “Martin Heidegger: the philosopher who fell for Hitler [print version: Hitler's philosopher]”, inThe Daily Telegraph (Review)‎^[1], London, pageR10:

     [P]hilosophers of the time [early 20th century] were primarily concerned with epistemology and the foundations of the sciences; they often spoke as if we were separated from the real world by a screen of "representations" or "sense-data"; they tended to regard our approach to the world as one ofdisinterested observation.

2. (proscribed)Uninterested, lacking interest.[from 17th c.]

   Synonyms:incurious,unaroused,unconcerned;see alsoThesaurus:apathetic

   Antonyms:curious,fascinated,interested

   • 1684,Contempl. State of Man I. x:

     Howdis-interested are they in all Worldly matters, since they fling their Wealth and Riches into the Sea.

   • 1936,Djuna Barnes,Nightwood, Faber & Faber, published2007, page43:

     Robin took to wandering again, to intermittent travel from which she came back hours, days later,disinterested.

   • 1967, Tommy Frazer,The Sun (Baltimore), "A 'Doctor' Of Karate", March 27

     Those spotted are usually taught so slowly that they growdisinterested and quit.

• The correctness of the use of this word with the meaninguninterested is disputed. Some reference works consider it acceptable,^[1] while others do not.^[2] The OED specifies that this is "Often regarded as a loose use."^[3] According to Macmillan Dictionary, "Many people think that this use of the word is not correct".^[4]