past master

noun

1. a person who is thoroughly experienced or exceptionally skilled in a profession, art, etc.: a past master at chess.
2. a person who has held the office of master in a guild, lodge, etc.

noun

1. a person with talent for, or experience in, a particular activitya past master of tact
2. a person who has held the office of master in a Freemasons’ lodge, guild, etc

A person who is thoroughly experienced or exceptionally skilled in some activity or craft. For example, We’re lucky to get Ella, because she’s a past master at fundraising. This expression probably alludes to the original literal meaning, that is, one who formerly held the post of master in a lodge or other organization. Although past mistress was used for an exceptionally skilled woman in the mid-1800s, it is heard less often today, master serving for both sexes. [Mid-1800s]