"Totenkopf" is German for "death's head" or skull and typically refers to a skull-and-crossbones image. During the Nazi era, Hitler's Schutzstaffel (SS) adopted one particular Totenkopf image as a symbol. Among other uses, it became the symbol of the SS-Totenkopfverbande (one of the original three branches of the SS, along with the Algemeine SS and the Waffen SS), whose purpose was to guard the concentration camps. Many original members of this organization were later transferred into and became the core of a Waffen SS division, the 3rd SS "Totenkopf" Panzer Division, which engaged in a number of war crimes during World War II.
Following the war, neo-Nazis and other white supremacists resurrected the Totenkopf as a hate symbol because of its importance to the SS and it has been a common hate symbol since. It is this particular image of a skull and crossbones that is considered a hate symbol, not any image of a skull and crossbones.