In Jewish law, food must be ritually cleaned and prepared in order to be kosher, or fit to eat. Today, it can also mean 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 can be used as an adjective, for example, "kosher meat." In the mid-1920's, the word took on a more general meaning, used to refer to anything that was acceptable.