Treif has come to be synonymous with food that is not kosher: pig, cheeseburgers, forbidden food additives etc. In fact, treif literally refers to an animal that is not kosher for one particular reason: that it was killed by a beast of prey, and not slaughtered according to the rules of shechitah.

The word itself means to rip, seize or tear. When Jacob sees his son Josephs bloodstained coat, he cries Tarof toraf Yosef, meaning Joseph is certainly torn to pieces and eaten by a wild animal, which is what Josephs brothers want him to think (Genesis 37:33).

On his deathbed, Jacob calls his son Benjamin, a zev yitraf (Genesis 49:27), a ravening wolf. In this, he is prophetically alluding, according to most commentators, to the martial prowess of Benjamins descendant, King Saul.

Recently, treif has come to be used in some circles in reference to books or ideas that the speaker deems to be unkosher, or not fitting to be consumed by religious Jews.

It is unfortunate, in my humble opinion, to use the legal term treif about a thought. The halachah is far less ready to legislate ideas than actions; indeed some would argue that it doesnt legislate them at all.