The meaning of "wolfshead"?

Hi all,

I've come across the term "wolfshead" in a song, but I can't find the meaning of the word. Does anyone know?

The song in question takes place in Mercia (which is a 1300 year old country), so I think it's an old word not used anymore.

Just to get the word in the right context, the lyrics go:

"By Offa's decree I am an outlaw, Branded wolfshead by my own king."

I think I heard of a type of punishment for criminals where they would be turned loose, but everyone who saw them where allowed to kill them without being ccused of murder. This may have been something made up, but this is my best guess at what wolfshead means.

Any help would be greatly appreciated, as google.com, dictionary.com and thesaurus.com have been to little help.

TIA

Devast8or