Hi! I am not sure if this question has been asked before, and in that case I would like to be linked to that thread. Anyway as a history student in Norway, I really wonder why the nazis had sculls on their hats, and why no one seemed to react on it. Was the sense of conformity that strong? For me it just seems like such a warning sign, with these sculls..
The use of the skull, or skull and crossbones also called the Totenkopf in German, as a military symbol dates back to well before the rise of the Nazis. It is still used today by a number of military units. The skull denotes death, either the death of the bearers enemy or that the bearer accepts the likelihood of death, preferring it to dishonorable defeat.
The death's head was a common symbol among European cavalry units in the 19th century. The post-WW1 German paramilitary units collectively called the Freikorps also used the Totenkopf as a symbol. Notably a number of early Nazi party members had served in the Freikorps including Ernst Rohm and Himmler. In that sense the adoption of the Totenkopf by the Nazis could be seen as a natural evolution.
Current units I know of that still use the skull in their unit emblem:
British Army Queens Royal Lancers : http://www.army.mod.uk/armoured/regiments/25401.aspx
USN Strike Fighter Squadron VFA-103 "Jolly Rogers" : http://www.vfa103.navy.mil/
There are numerous others.
As the other commenters have said, the scull and crossbones were way older.
The SS, however, had it as their symbol since their foundation in 1925, and before that the "Stroßtrupp Adolf Hitler" had a skull on their hats.
They had it as there symbol because there were Freikorps (as /u/NoAstronomer said, right wing paramilitary units formed by veterans) after the First World War with that symbol. But why did those people have it?
They called themselves Freikorps after German volunteer units in the Napoleonic Wars. One of these Freikorps, the most famous, was called Lützowsches Freikorps. The volunteers had to provide their own uniforms, so for easier dye-ing, they chose black and not prussian blue as their color (note that the uniforms of the SS were also black). Some soldiers took the design for their hats from another Unit, the "Schwarze Schar", which more or less was a Freikorps itself, afterwards the Schar was made a regular part of the Prussian army, where the skull, indeed the whole set of black uniform with silver skulls on the headgear, lifed on until the end of Prussia. [I'm afraid that I cannot tell where that unit had the skull from] Seemingly as this book argues, the skulls came as a reminder of their motto "Sieg oder Tod", "Victory or Death".
So in short, the SS had it from the Stroßtrupp Adolf Hitler, which had it from the Freikorps after WWI, which had it partly from the antinapoleonic Freikorps and partly from a Freikorps turned Prussian Army unit.