From the second edition (1989):
(ʃnʊk) Also schnuck, shnook. [app. Yiddish: perh. repr. Yiddish shnuk snout, or f. G. schnucke a small sheep.]

A dupe, a sucker; a simpleton, a ‘dope’; a pitiful wretch.

[1943 S. J. Perelman Let. 7 Apr. in G. Marx Groucho Lett. (1967) 190 It's the story of a small schnükel of a barber who accidentally brings a statue of Venus to life.] 1948 H. L. Mencken Amer. Lang. Suppl. II. 757 Schnuck‥, a customer easily persuaded, a sucker. 1955 N. Mailer Deer Park xii. 136 I'd be making a stinking seven hundred and fifty a week now like all those poor exploited schnooks. 1959 R. Chandler in Gardiner & Walker R. Chandler Speaking (1962) 262 Why does he want to see me so badly that he has to send a couple of shnooks after me? 1964 S. Bellow Herzog 29 This shnook of a chiropodist—what a hellcat he married. 1975 A. Bergman Hollywood & Le Vine xiii. 187 It was all pretty fascinating for a Sunnyside schnook like me. 1980 W. Safire in N.Y. Times Mag. 2 Aug. 8 To be self-conscious about the possibility of error‥is to be a nerd, a schnook and a wimp.