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A Jewish foreskin or Jew's foreskin is always off, therefore the expression is a very definite one about being off on one's way or just in a generally off condition.
It is time that i was on my way, so I am off like a Jewish foreskin. (or Jew's foreskin)
by Roderick O'Donnell July 30, 2006
About to leave or take off.

Refers to a circumsized penis, where the foreskin has been removed or taken "off".
It was great seeing you guys, but I really have to catch my bus, so I'm off like a Jewish foreskin.
by andycash May 15, 2006