The person at the back of an anal sexual encounter. While the term is most often used as a derogatory reference to homosexual men, it can equally well be applied to lucky bastard straight guys who find women who will let them fuck them in the ass.
1. Although few people know it, the term derives from the name Germans called Canadian Expeditionary Force soldiers during WWI ("Sturmmann"). Certain divisions of German soldiers, both in WWI and WWII (Nazis), would later call themselves this name. Its meaning is exactly what you would expect--troops capable of quickly and efficiently overcoming opposing forces.