'correctly' englishing Yiddish is much like targeting that ol' rolling donut

Yes, Yiddish שםאָק (shmok) > English schmuck /'ʃmʌk/ 'prick; fool'. You see the same thing in Yiddish פאָץ (pots) 'penis' > English putz /'pʌts/ 'prick; fool'. I have seen the same sort of thing in German where English punk (either the music or one of its fans) > German Pank.

It is interesting that in Yiddish these two words for the male genitalia have become so much more used for foolish persons that new terms for penis have developed: shmekl and petsl, with diminutive suffix -l.

Ceci n'est pas un seing.