Scutum, not Scrotum. A scutum is a Roman shield, and the arm is named for the constellation of Scutum in that area.
Scutum was originally named Scutum Sobieski, the Shield of Sobieski. Jan Sobieski was the Polish king who, with his army, came to the aid of the Viennese and lifted the Siege of Vienna by the Turks back in the 1600s. A Polish astronomer named the constellation in his honor.
http://chandra.harvard.edu/photo/constellations/scutum.html