The Red Shirts were a nationalistic group formed with the intention of eliminating foreign rule in Italy during the 19th century. This group was led by Giuseppe Garibaldi.
"Red Shirt" is also used to describe the doomed security officers who accompanied Captain Kirk of Star Trek in landing parties. They would frequently meet sudden, grisly death as a way to establish danger. The term has entered the popular lexicon to describe minor characters whose sole purpose is to be killed. See also Homeric spear carrier.