Illth is the name John Ruskin
gave to the goods which an economy
produces that do not contribute to human welfare. (They may help the economy, in that they put more money into circulation and create jobs. But the ownership of these items doesn't help anybody.)
He used the term in contrast to wealth, those items which do make us better off, such as capital, useful manufactured goods, food, natural resources, ect. As I understand it, art would be a type of wealth also.
He also makes the point that whether something is wealth or illth depends not only on what it is, but how it is used. He gives the example of wine:
Thus, wine, which the Greeks, in their Bacchus, made rightly the type of all passion, and which, when used, "cheereth god and man" (that is to say, strengthens both the divine life, or reasoning power, and the earthy, or carnal power, of man); yet,
when abused, becomes "Dionysos," hurtful especially to the divine part of man, or reason.