The term "hobknocker" became popular when it appeared on the TV show iCarly. It's true meaning is not really known, but it was implied that it refers to someone who does gross and illegal things in public.
A hobnocker was a name term used in the 1800's. Hobnocker was used in the Great Depression to identify tramps and hobos. It referred to those who they considered to just be roaming around and take on very little responsibility.
Hobnocker is a slang term that actually has a lot of different definitions. For example, it can be used to describe the act of doing something illegal or inappropriate. It can also be used to describe a tramp or a hobo.
The term hob knocker was used on the television show "iCarly" by singer David Archuleta. While there has been rumors and speculation indicating it means something dirty, it appears that it is just an insult that has no true meaning.