Green’s Dictionary of Slang

(US) in burlesque, a slur aimed at a Jewish comedian, who has worked the Jewish resorts of the Catskill Mountains known as the ‘Jewish Alps’ or ‘Borscht Belt’.

1938

H.M. Anderson Strip Tease 44: The straight man always has one epithet with which to squelch a comic...‘mountaineer.’ Many of them got their training in what Variety calls the broscht [sic] circuit, the Jewish boarding houses in the Catskills.