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One who is exceptionally skilled or experienced in a particular activity, craft, or field. Typically followed by "in/of/at (something)." Not to toot my own horn, but I consider myself a bit of a past master in cooking. I don't blame you for feeling that way—Jen is a real past master at eroding your self-confidence. He's always been a past master of getting by with doing as little work as possible.
*past master (at something)
Fig. someone proven extremely good or skillful at an activity. (*Typically: be ~; become ~.) Mary is a past master at cooking omelets. Pam is a past master at the art of complaining.
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]