Usually specific to soldiers, or other people who are forced to live in close proximity for long periods of time without either sexual activity or privacy, a whackatorium is a small shack made out of blankets, ponchos or poncho liners that is erected so that the builder can masturbate in relative peace. It is often explained away as a means to block light, or provide privacy to change, despite being erected almost exclusively around one's sleeping quarters.
syn: jack shack