A dirty person with a dog who took the rebellion thing a little too seriously.
Their hobbies typically include: freight train hopping, dumpster diving, asking you for money, and smoking crack.
See that junkie in the Amebix shirt with the incredibly large backpack? He's what we like to call a crusty. He probably ran away from home because his parents wanted him to go to college.
Entirely ratchet in every sense of the word. Someone who is absolutelydisgusting and whom you would not touch with a ten foot pole. Avoid crusty strangers with douchey selfies.
Always negative connotation unless used to describe pizza.