What is the difference between disinterested and uninterested?

Disinterested means impartial — having no stake in the outcome. Uninterested means bored, or not paying attention.

A disinterested judge is exactly what you want: someone with nothing to gain either way. An uninterested judge is not paying attention, which is a serious problem.

The distinction is worth preserving because disinterested carries a meaning nothing else expresses in one word. "Impartial" comes close, but "a disinterested party" is a fixed and useful phrase in legal and financial contexts, where it means specifically that the person has no financial or personal interest.

The complication is historical. Disinterested originally meant "not interested" in the seventeenth century, and uninterested originally meant impartial — the two swapped over time before settling into their current split. So the modern merger is arguably a return rather than a decline, which somewhat weakens the case for treating it as a fresh corruption.

Nonetheless, in current edited English the distinction is expected, and using disinterested to mean bored will be noticed by careful readers.

The same dis- and un- pattern produces other pairs worth knowing. Disorganised describes a state of disorder; unorganised simply means not organised into a structure. Disassemble means to take apart; dissemble means to conceal your true motives, and is unrelated despite the near-identical spelling.

A reliable memory hook: disinterested relates to having no interest in the financial sense — a stake. Uninterested relates to interest in the everyday sense — curiosity.

If you mean bored, the safest word is simply uninterested, or better, bored.

One related trap: nonplussed means bewildered, not unbothered — though the second sense is now so common in North American usage that dictionaries record both, and the word has arguably become unusable in precise writing for that reason.