Caked-on dirt is a type of dirt that has been stuck to a surface for a long period of time and is difficult to remove. The term 'CRUD' is often used to describe this type of dirt, which is an acronym for 'Caked-on Residue Under Dirt'. This type of dirt is often found on surfaces that have not been cleaned regularly, such as the inside of a car or the bottom of a shoe. It can also be found on outdoor surfaces that have been exposed to the elements for a long time.