Other forms: koshers
In Jewish law, food must be ritually cleaned and prepared in order to be kosher, or fit to eat. Today, the word can also describe anything that is proper or legitimate. Is it kosher to date your best friend's ex? Depends on who you ask.
The word kosher, literally meaning "clean" or "pure," refers to food that has been ritually prepared or blessed so it can be eaten by religious Jews. It comes from the Hebrew word kasher, meaning "proper" or "lawful," and became common in English in the mid-19th century. It is usually used as an adjective, as in "kosher meat." In the mid-1920s, the word took on a more general meaning, used to refer to anything that was acceptable.