When you make something less valuable or less important, you devalue it. You might devalue your car by spray painting it black and letting your dog chew the upholstery.
Economists and business reporters often use the verb devalue when they talk about the currency — or money — of different countries, and what it's worth in comparison to other currencies. Sometimes countries will devalue their currency, for example, to get more people to buy their exported products. But you can also devalue a relationship by neglecting it, or devalue a diamond ring by chipping it. Devaluation came before devalue, with a Latin root of valere, "be strong or be of value."