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.
My family doesn't give me any privacy so I have to go to my Jack shack in the woods when I want to rub one out or smoke a joint.
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
He had only been in Iraq for a week, but he had alreadybuilt an intricate and comfortable jack shack.