Something is non-fungible when it is unique, and not directly interchangeable with anything. That can be due to its properties, production date or any number of unique aspects. A house, diamonds, trading cards or artwork are example of non-fungible things. Conversely, currency is fungible because $10 can be swapped with $10 and no-one is at a disadvantage.
“My wife is non-fungible. Ain’t no-one like her in the world.”« Dictionary Menu