Not kosher. Plain and simple. Doesn't mean anything else. This can mean literally not kosher, as in not adhering to Jewish dietary laws, or not kosher in the more metaphorical sense, as in not legitimate. The latter usage definitely falls into the category of slang.

Sometimes spelled 'treyf'. It's a transliteration from Yiddish.

also treifah
noun, uncountable and countable
1) an unreasonable Jewish contempt for food, culture, nationality, religion, race etc

noun, countable
2) someone who always complains about anything in sight