The Prussians don't fire that fast - Settle down, not so fast. There are several sources suggested for this one. Most probably it stems from around the time of the Austro-Prussian war in 1866. Appearantly officials in Vienna exclaimed it in disbelief after they received word of the new Prussian Dreyse eedle gun infantry rifles which were able to fire a lot faster than the regular muzzle-loaded muskets. Another possible explanation is that around that time the Prussians didn't instantly shoot deserters. They beat them half to death, but then nursed them back to health. Either way, it's a very common saying.