someone who was formerly a master

In Freemasonry, Master Masons who have "been through the chairs", that is, who have completed a term of office as Worshipful Master of a Lodge, are ranked as Past Masters in the Craft as a whole, and in Grand Lodge in particular. They alone are qualified to participate in certain work of a Lodge. In addition, Lodges commonly appoint Past Masters to certain offices or functions: for example, it is practically essential for a Director of Ceremonies (also known as a Ritualist in some jurisdictions) to be qualified as a Past Master, or he could not attend all the ceremonies; and a Lodge's by-laws often require a proportion of the Lodge's committee members to be Past Masters.

Past Master is a novel by science fiction writer R. A. Lafferty. It was first published in 1968, and was nominated for the 1968 Nebula award (Rite of Passage won) and the 1969 Hugo award (Stand on Zanzibar won)http://members.tripod.com/templetongate/hugo-nebula5370.htm "Hugo & Nebula Awards"; http://www.scottedelman.com/cordwainersmith.html "2002 Cordwainer Smith Rediscovery Award".

someone who has long and thorough experience in a given activity