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.
Literal: Pork fried rice is treif.
Metaphorical: Mike's urine test was treif. He use'd Bill's piss because he was tokin' last Tuesday and didn't want it to show up.
noun, uncountable and countable
1) an unreasonable Jewish contempt for food, culture, nationality, religion, race etc
2) someone who always complains about anything in sight
(at a sushi bar in Okayama, Japan) We’ve got a treif from Rochester, NY.