The Hun were a warlike race of people from central Asia
Attila the Hun being the most famous one .
They appeared around Europe during the 4th century and did a bit conquering of the Germans and various others ..
And Wikipedia says ......
On July 27, 1900, during the Boxer Rebellion in China, Kaiser Wilhelm II of Germany gave the order to "make the name 'German' remembered in China for a thousand years, so that no Chinaman will ever again dare to even squint at a German". This speech, wherein Kaiser Wilhelm invoked the memory of the 5th-century Huns, coupled with the Pickelhaube or spiked helmet worn by German forces until 1916, that was reminiscent of ancient Hun (and Hungarian) helmets, gave rise to later English use of the term for the German enemy during World War I. This usage was reinforced by Allied propaganda throughout the war, and many pilots of the RFC referred to their foe as "The Hun". The usage resurfaced during World War II.