You are supposed to be courteous when you drive in Germany. The law says so! Wave your fist or shout at another driver and you can face a stiff fine!
About the only thing that you can do is to mouth something silently. The courts have decided that you are allowed to assume that the target of your ire is not a lip reader and so you can say what you like.
This is all taken quite seriously and if you do wave your fist a lot, you may find that half a dozen Germans have phoned the police and you are in trouble