Many object to the use of disinterested to mean “not interested, indifferent.” They insist that disinterested can mean only “impartial”: A disinterested observer is the best judge of behavior. However, both senses are well established in all varieties of English, and the sense intended is almost always clear from the context.
Examples from the Web for disinterestedness
Disinterestedness not the differentia of aesthetic pleasure, 37 et seq.The Sense of Beauty|George Santayana
Dwelling in a house where disinterestedness and noble labor were as daily breath, she had great opportunities.The Works of Whittier, Volume VI (of VII)|John Greenleaf Whittier
A child is more impressed with a single example of disinterestedness, than with a hundred admonitions on the subject.The Travellers|Catharine Maria Sedgwick
Nothing can go beyond this; it proves her beauty and her disinterestedness.
But, in France at least, these critics were the first to render justice to his learning, his talents and his disinterestedness.
British Dictionary definitions for disinterestedness
Word Origin and History for disinterestedness
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.