Soldiers in Hitler's
storm trooper army, SA during the Nazi regime. They were called this because of the colored shirts they wore.
To call someone a brown shirt is to call them a
Nazi soldier, usually hinting that they oppose human rights or freedom. This is a very serious insult, and in forums is used to Flame someone.
Hitler used many different forces to accomplish his goals when he rose to power. One of these forces were the SA, or Storm Troopers. The SA soldiers were called
Brown Shirts.