Many martial races/people had unwritten laws = customs and rules that were unwritten.

The Japanese Bushido http://en.wikipedia.org/wiki/Bushido,
The Albanian Kanun http://en.wikipedia.org/wiki/Kanun, and
Pashtun Pashtunwali http://en.wikipedia.org/wiki/Pashtunwali.

They didn't need to be written down before the arrival of outsiders because the locals were brought up with it and there wasn't a need to. Generations passed it on from birth. Once outsiders came into contact with these groups it had to be written down so they can be communicated efficiently with these foreigners.

مننه وروره

Brother just want to add one more thing. When historians say unwritten, it is actually to emphasize on the importance of these laws. These laws are so important and part of the social structure that they don't need to be written. They are part of life and expected of everyone.