A jack shack is a place like a seedy massage parlor or "modeling studio" or "gentlemen's spa" where you can get a hand job for a reasonable price, and little fear of getting arrested for soliciting.
A shed, shack, camping shelter, tent or other shelter which provides protection from view and privacy, when used by the inhabitant (often a teenager) to masturbate, smoke marijuana, or drink alcohol without being discovered or disturbed.
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 jack shack 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: whackatorium