uninterested adj.

prefix of negation, Old English un-, from Proto-Germanic *un- (source also of Old Saxon, Old Frisian, Old High German, German un-, Gothic un-, Dutch on-), from PIE *n- (source of Sanskrit a-, an- "not," Greek a-, an-, Old Irish an-, Latin in-), combining form of PIE root *ne- "not." Often euphemistic (such as untruth for "lie").

The most prolific of English prefixes, freely and widely used in Old English, where it forms more than 1,000 compounds. It underwent a mass extinction in early Middle English, but emerged with renewed vigor 16c. to form compounds with native and imported words. It disputes with Latin-derived cognate in- (1) the right to form the negation of certain words (indigestable/undigestable, etc.), and though both might be deployed in cooperation to indicate shades of meaning (unfamous/infamous), typically they are not.

It also makes words from phrases (such as uncalled-for, c. 1600; undreamed-of, 1630s; uncome-at-able, 1690s; unputdownable, 1947, of a book; un-in-one-breath-utterable, Ben Jonson; etc., but the habit is not restricted to un-; such as put-up-able-with, 1812). As a prefix in telegramese to replace not and save the cost of a word, it is attested by 1936.

"cause to be interested, engage the attention of," c. 1600, earlier interesse (1560s), from the noun (see interest (n.)). Perhaps also from or influenced by interess'd, past participle of interesse.

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]