Green’s Dictionary of Slang

(US) an expert, a connoisseur.

1950		Jewish Standard (Toronto) June 36 [advert] Both of these epicures aptly describe the ‘pareve’, easy-to-fix dish that wins the applause of every ‘mayvin’ [OED].
1952		in N.Y. Times Mag. 21 Sept. 58/3: The most trying type [of customer] of all [...] is the ‘mayvin.’ The word is of Yiddish origin [and] has entered the language [W&F].
1960		Taunton Courier 23 July 3/6: American slang [...] A niff-naws or a hair-bag. The moll-buzzers bullskate and the mayvins yentz.
1968		L. Rosten Joys of Yiddish 226: Mavin was recently given considerable publicity in a series of newspaper advertisments for herring tidbits. ‘The Herring Mavin Strikes Again!’ proclaimed the caption.
1995		R. Campbell Wizard of La-La Land (1999) 14: Medical mavens hanging around gentry’s Coffee Shop — where there was always at least one expert on any human belief, profession, pastime, or enterprise — maintained that something had shocked his pituitary.