Synonyms for disinterested

Most common

1. Uninterested — synonym of uninterested (“not interested; not concerned; indifferent”)

2. Impartial — having no interest or stake in the outcome, and no conflicts of interest; free of bias, impartial

Other

unselfish, generous

adj. 1
Having no interest or stake in the outcome, and no conflicts of interest; free of bias, impartial.
adj. 2
Synonym of uninterested (“not interested; not concerned; indifferent”).

Disinterested or Generous?

Disinterested suggests rather the thought of one's own self-denial; generous, of one's hearty interest in another's welfare or happiness.

J. C. Fernald, English Synonyms and Antonyms (1914)

apathetic It's said that the youth of today are apathetic.
unconcerned Tom said that he thought that I seemed unconcerned.
listless I feel listless and have a throbbing headache.
lackadaisical His lackadaisical attitude to studying explains his poor grades.
impartial I doubt very seriously that Tom will be impartial.
dispassionate Tom always sounds so dispassionate, no matter what he's talking about.

Use in printed books, per decade from 1850 to 2019.

4 letters: cold¹, dull¹
5 letters: jaded²
6 letters: candid², supine², torpid²
7 letters: liberal³, neutral², passive², unmoved², languid²
8 letters: clinical³, generous³, detached², unbiased³, uncaring³, listless²
9 letters: objective³, impartial³, equitable⁴, apathetic⁴, lethargic³, unselfish, impassive³, incurious, unaroused, unpartial, unbigoted
10 letters: nonchalant³, phlegmatic³, insouciant³, spiritless, unpartisan
11 letters: unconcerned³, pococurante, nonpersonal
12 letters: unprejudiced
13 letters: dispassionate⁴, lackadaisical⁵, imprejudicate
14 letters: unenthusiastic⁶, unprepossessed

The small number after each word is its number of syllables (jaded² = 2 syllables).