The constant misuse of disinterested for uninterested is breaking down a very useful distinction of meaning.
To be uninterested is to be lacking in any sense of engagement with the matter:
Sallie is uninterested in algebra.
To be disinterested is to lack bias:
Let the company call in a disinterested mediator to settle the dispute.
The use of disinterest as a verb should probably be avoided:
Her husband tried to disinterest her in taking the course in German.
Better: Her husband tried to discourage her from taking the course in German.
If the person you are describing is not interested in something, use uninterested.
Save disinterested for the judge.
Here’s a quote from a NY Times article:
The passing of the old sense of interest took disinterestedness over the side along with it. Usage critics exaggerate when they lament the disappearance of the sense of disinterested that means, roughly, ”impartial.” That meaning still accounts for a majority of its uses in the press.
But disinterested isn’t a word that comes up much when we try to define political virtue. There’s no place to stand that’s free from what William Dean Howells called ”the sordid competition of interests,” now that interest itself has been given so broad a charter.