What is the Scrum Master a Master of?

Scrum master if more of servant leader to the scrum team. He is masters understanding and mindset of scrum rather than master of the team. 

Master of Scrum....

The word master has different measnings, and I think it is most of the time misinterpreted for "guru", as apposed to "apprentice".

Master, in the sence of Scrum Master, is not an indication of Mastery (like containing all the ingredients to be the master, the guru), but should in my view be interpreted as the other meaning: teacher, instructor, leader.

So Scrum Master is a teacher of Scrum. He or she will lead the team to adoption of the important principles of Scrum.

Unfortunately, often the master of Scrum only, which for me, qualifies as a deadly sin.

They are the master of their own domain.  (Any one get the Seinfeld reference?)

Their domain is Scrum.  As @Xander Ladage points out though, this doesn't mean they know everything there is to know.  I see it more as subject matter expertise.  We work in a skilled trade.  If you look at other skilled trades there are "master" designations there too such as Master Plumber, Master Mechanic, or Master Electrician.  We have all heard the job titles Webmaster and Postmaster. There are Masters degrees available in just about any discipline.  Military have ranks like Master Sergeant.  All of these are used to indicate a higher level of discipline and knowledge.

Personally, I view myself as someone that is extremely interested and invested in the agile mindset and craves to know as much as I can about it. I am continually searching for new knowledge, new ideas, new research, new situational experience that will aid me in my quest to help others be better than they think they can.