past master

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One who has formerly held the position of master in an organization, especially a social one such as a lodge or club.
• noun One who is thoroughly experienced and skilled in a particular craft or activity.

from The Century Dictionary.

• noun See passed master, under master.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun freemasonry Someone who has previously been a master at a given lodge.
• noun One who has an extremely high level of ability or knowledge within an area of achievement or expertise.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun someone who was formerly a master
• noun someone who has long and thorough experience in a given activity