Someone who was formerly a master.

Roget 700: proficient, expert, adept, dab; dabster^†, crackerjack; connoisseur etc. (scholar) 492; master, master hand; prima donna [Sp.], first fiddle, top gun, chef de cuisine, top sawyer; protagonist; past master; mahatma.    picked man; medallist, prizeman^†.    veteran; old stager, old campaigner, old soldier, old file, old hand; man of business, man of the world.    nice hand, good hand, clean hand; practiced hand, experienced eye, experienced hand; marksman; good shot, dead shot, crack shot; ropedancer, funambulist^†, acrobat; cunning man; conjuror etc. (deceiver) 548; wizard etc. 994.    genius; mastermind, master head, master spirit.    cunning blade, sharp blade, sharp fellow; jobber; cracksman etc. (thief) 792^†; politician, tactician, strategist.    pantologist^†, admirable Crichton, Jack of all trades; prodigy of learning.    ... show more

Someone who has long and thorough experience in a given activity.