Douglas Harper's Etymology Dictionary
schm-
substituted for the initial sound of a word and reduplicated with it to convey derision (as in "Oedipus schmoedipus" in the punchline of the old joke about the Jewish mother and the psychiatrist), 1929, from the numerous Yiddish words that begin with this sound.
Wiktionary
schm-
pre. Used to form a reduplicated rhyming compound of any word in order to express disparagement, dismissal or derision