Used in common phrases by rednecks/ white trash. It means to be madder than a wet hen, and when a hen gets wet; it gets really mad. The term cannot be said the way it looks though. You have to break it up all redneck like. Such as the example.
A way to say 'It doesn't matter' in the manner of the wrestler and actor, 'The Rock'. Said in a deep and slighly aggressive tone. Dothun madder can be used as a more polite form of 'shut up'.