Godwin's law is an extension of the Reductio ad Hitlerum which is part of the Usenet folklore. In 1990, Mike Godwin stated the following rule of thumb: "The longer a discussion on Usenet, the more likely it is to compare it with the Nazis or Hitler" 1
Explanations
This “law” is based on the assumption that a Usenet discussion that lasts over time gradually leads people to warm up and replace arguments with insults, or to use extreme arguments or analogies. Since Nazism is often considered the worst of ideologies, any comparison of the subject or of a speaker with a movement of this kind is considered to be a sign of the failure of the discussion, at least if the starting point was very far away. We therefore believe that it is time to close the debate, from which nothing relevant will emerge, to start on a sound basis. We then say that we have reached the “Godwin point” of the discussion.
The following: http://fr.wikipedia.org/wiki/Loi_de_Godwin
ps: I feel like we're going to godwinate a lot now