past master
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]