past master

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

noun

A person with a high degree of knowledge or skill in a particular field: ace, adept, authority, dab hand, expert, master, professional, proficient, wizard. Informal whiz. Slang crackerjack. Chiefly British dab². See ability/inability.

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]

The noun has 2 meanings:

Meaning #1: someone who was formerly a master

Meaning #2: someone who has long and thorough experience in a given activity