1. One who is filthy, grimy, and fiends for an object(s); possesing a strong desire for drugs, money, any sort of vice or source of pleasure.
2. To be described as a fiend one must be both crust covered and fiendly beyond normal human levels of fiendliness or crustendom.
Child: Hey look at that guy over there; he is really strange.
Mom: Son watch out for crusty fiends like that; they can be dangerous.