::Marine climbs rope at the end of the obstacle course and slaps the wooden beam at the top, yelling "Oorah!" ::
"Son, you good to go?"
"Grab that pack!"
Started with Marine Amphibious units during the Korean War in 1953. Many Marines had to be delivered to shore by means of submarine. On US Submarines the klaxon alarm sounds when the vessel is about to dive with its classic "Aoogah" sound. When Marines on the submarine heard this distinctive sound it usually meant that they were about to go into battle on shore very soon. It soon became shortened to "Oorah" and signified that a Marine was battle-ready.
Pvt. - Oorah, sir!