Beerwolf is a concept introduced by Martin Luther (in a 1539 debate) that Luther uses to describe the Pope and Holy Roman Emperor Charles V. In the context of resistance theory, a "Beerwolf", "in contrast to a mere tyrant, not only broke the law, but overturned the entire moral order upon which it is based. All the subjects of such a ruler ... had the right to resist and even to kill him and all his supporters.
The significance of the term lies in the fact that, for most of his life, Luther held that no subject could actively resist his secular ruler, an issue of obvious significance in a time when many rulers in the German lands and their respective subjects held competing religious beliefs. The concept of Beerwolf marked Luther's final, and most extreme, position on resistance theory, as it relied on natural law (specifically, what would later come to be called Hobbes' right to self preservation) instead of earlier and more limited rights to resistance that Luther had accepted as flowing from German constitutional law.
The concept of just rebellion that the term Beerwolf introduced was subsequently developed by fellow Protestants who faced a similar situation in France, the Huguenot Monarchomachs.