The story
From the Yiddish; one of several words Anglicized and popularized by the original writer of MAD Magazine, Harvey Kurtzman. The word comes from shlogn ("to hit") with the prefix far- which often indicates the one so described is taking on the quality named. Thus, in Yiddish it means something that is old, battered, or junky. Because many American Jews had only a sketchy knowledge of Yiddish, and due to the vagaries and difficulties of transliteration, words changed in spelling and consequently in pronunciation. The word should have been transliterated as "farshlugginer" for a more accurate pronunciation.
Etymology from Wiktionary, CC BY-SA 4.0.