Bailiwick: someones special area of knowledge

Your bailiwick is your area of expertise. 

Bailiwick comes from the word bailiff (an officer of the court) and the Old English word for village, win. 

Algebra was not my bailiwick in high school, or in college.  Made me feel pretty dumb, actually. 

Later, I found out algebra is only useful if you are going to become an algebra teacher.  It only exists for itself.  Algebra is one of the Kardashians of the math world: