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The phrase used when someone does something that most people don't agree with; socially unacceptable; politically incorrect.

This phrase cannot be used in reference to dill pickles, because I'm pretty sure those are always kosher.
Jack: John, are you stealing those little kids' lunch money again? You're like 40 man you need to stop that.

John: (Looks down at his feet with shame)

Jack: Not kosher man, not kosher at all
by Andy Hesterman December 28, 2007
The noble founder of ontd_anas
Not Kosher: Don't forget to douche!
Aurosan: Douching is bad for you!
by notkoshersbigfan December 20, 2007