Fundamental Principle of Fiqh #5

"Custom is a source of law." العادة محكَّمة

In fiqh, custom (action, behavior, or even words common to a certain people) is considered as unwritten law as long as it meets the following criteria:
- It should not contradict the shariah; e.g., alcohol should not be made permissible.
- It should not contradict well known meanings; e.g., the word "disbeliever" should not be used to refer to a believer.
- It must be practiced or adopted by a large number of people.
- It must be in current usage. If the custom in question is a word or action that has been changed from its original meaning, then the current usage and meaning is considered.

This important principle finds application in many situations. It instructs us that a word or a physical gesture or sign could be considered by the shariah as either acceptable or offensive, depending on how it's perceived by the culture of the people involved.