In modern English, the word interested has two different meanings.
The first, older one is approximately “having a personal involvement in”, as in “He is an interested party in the dispute”.
The second and later, but now more common, meaning is “demonstrating or experiencing curiosity about, enthusiasm for”, as in “He is very interested in cricket”.
In all languages there are many words that have more than one meaning – this is a common state of affairs.
Most English speakers, for example, can instantly think of a number of different meanings for the words common, state and affairs which I have just used. Confusion never seems to occur, largely because the context will normally make it obvious which sense of the word is intended.
According to dictionaries, the two meanings of interested have different negative forms. The negative of the first sense, as in “He is an interested party in the dispute”, is disinterested: “He is a disinterested person and therefore able to be more objective about it”. Disinterested, then, is roughly equivalent to “neutral, impartial, not biased”.
The negative form of the second, more usual meaning is uninterested, as in “I am uninterested in all sports”. So uninterested is roughly equivalent to “bored with, feeling no curiosity about, not interested”.
These days, interested in its original meaning is a rather unusual, learned, formal word in English.
Most people, if they wanted to convey this concept in normal everyday speech, would probably say something like “not neutral”, or “biased” or “involved”.
This unfamiliarity with the older meaning of the word interested has led
to many people now using disinterested with the same meaning as uninterested: “I admit that I am disinterested in cricket”.
Some people object to this, claiming it is an ignorant misuse of the word, and that a very useful distinction is being lost.
But we should not be surprised at this development. The prefix dis– is commonly used in English to turn positive adjectives into negative ones. The negative forms of pleasing, honest and agreeable are displeasing, dishonest and disagreeable.
And it is important to point out that there are actually some benefits to be gained from this new use of disinterested: we now have a single-word noun corresponding to the adjective.
Before, we did not have in English a word such as uninterestedness or uninterest, so we had to use rather clumsy longer expressions such as lack of interest. Now, though, we can say things like “John showed considerable disinterest in the game of cricket”.
There also seems to be a tendency for speakers today to make a small difference of meaning between the two forms.
This is something that also very often happens to synonyms – they very rarely stay complete synonyms. So disinterested often appears to be stronger in meaning than uninterested, with disinterested indicating a strong lack of interest, while uninterested refers to simple apathy or indifference.
Also, the use of disinterested in this relatively new way does not cause any difficulties, because the context nearly always makes it clear which meaning is intended.
If we never get confused about the two different meanings of interested, why should we be confused if disinterested also has two meanings?
“The school children looked very disinterested” is not likely to be ambiguous, nor is “As an arbitrator, they needed someone who was completely disinterested”.
In Classical Latin, the word ambo meant ‘both’, as in ambidextrous. Agere meant ‘to act’, as in agent. The combined Latin form ambigere meant ‘to quarrel, be undecided, doubt’. The adjective derived from this, ambiguus, meant ‘undecided, doubtful, hybrid, indeterminate, unreliable’ and, like the English word, ‘with more than one meaning’.