Imagine something really disgusting — crusty, dirty, falling apart — you know, like zombie flesh. If it’s shoddy, gross, and really, really cruddy, it can be called mangy.
If you want a really insulting adjective, you can’t do better than the word mangy. A mangy dog has a skin disease carried by mites that causes the animal to lose its hair and develop crusty patches on its body. Or, if you own a mangy sweater, it is probably threadbare and stained. As you can imagine, you should probably think twice before calling a person mangy!