The term "crusty" is an adjective that describes something or someone with a rough, hard exterior. It can be used to describe a person who has a surly, unfriendly demeanor and whose behavior is difficult to deal with. The term can also refer to a dry, crumbly texture, such as in bread or pastry. In the context of geology, "crusty" refers to the Earth's outermost layer - the lithosphere, which includes the crust and the uppermost part of the mantle.