From <a href=“http://www.straightdope.com/mailbag/myiddish.html">"Yiddish, shmiddish–why do we repeat a word but start it with “shm-”?”</a>
I’m reasonably certain that “schmuck” is not of Yiddish origin. I suspect Italian slang, or something, but I have been unable to find it in any good Yiddish lexicon.
(And I don’t count Leo Rosten’s “Joys of Yiddish” as a GOOD Yiddish lexicon.)