past-master

Noun

past master (plural past masters)

1. One who has an extremely high level of ability or knowledge within an area of achievement or expertise.
   • It required a deft hand and deep knowledge of the barbaric mind effectually to handle such diverse weapons; but he was a past-master in the art, knowing when to conciliate and when to threaten with Jove-like wrath.1900 April 7, Jack London, “The Son of the Wolf”, in The Son of the Wolf: Tales of the Far North, Boston, Mass.; New York, N.Y.: Houghton, Mifflin and Company […], →OCLC, page 24

   Synonyms:

   expert, guru, adept, ace, artist, buff, crackerjack, crack hand, dab, dab hand

2. Someone who has previously been a master at a given lodge.