A shack is a small, rundown building used as a shelter. To shack is to live somewhere. If you tell your parents you want to shack up with your best friend, prepare to get grounded.
A shack is a tiny, crude shelter that one person might be living in. It’s not well maintained, and it probably has peeling paint and a leaky roof. A shack is a big step below a house and a small step above a refrigerator box. To shack is to live somewhere, especially somewhere that's not nice. Shacking up is kind of like crashing on someone’s couch, or living somewhere temporarily.