someone who has long and thorough experience in a given activity someone who was formerly a master
If you are a past master at something, you are very skilful at it because you have had a lot of experience doing it. He was a past-master at manipulating the media for his own ends She is an adept rock-climber and a past master of the assault course. = expert. someone who is very skilled at doing something and has done it many times before = expert past master at (doing) sth