Even kids jumping off of playground equipment scream Geronimo before they let loose, but have you ever stopped and wondered why people do this? Well, Mental Floss has the story:
In the early 1940s, the United States Army was testing the idea of parachuting from planes as a way of deploying troops. The first group to really experiment with it and begin developing paratrooper techniques was a unit of 50 men known the Parachute Test Platoon.
...When training was done for the day, the troops liked to loosen up and cool down a little. Usually, most of the guys went to the air-conditioned Main Post Theatre in the evenings to see whatever movie was playing. One night in August 1940, that movie happened to be the Paramount western, Geronimo, about the Apache chief.
Read the rest of the story over at the link.