Green’s Dictionary of Slang

1. (US, also shack) a person with whom one has a sexual relationship; also attrib.

1953		L. Uris Battle Cry (1964) 192: The men drank and slept with their shackups.
1969		J. Crumley One to Count Cadence (1987) 62: His steady shack, Dottie the bow-legged whore.
1991		(con. 1920s) O.D. Brooks Legs 225: You’ll get pieced off like a shack-up broad.

2. an act of sexual intercourse; also attrib.

1953		W. Eyster Far from the Customary Skies 58: I’d sure like one shack-up job before we go across.
1967		E. Shepard Doom Pussy 168: He’s just come back from a shack-up with her somewhere.
1977		B. Davidson Collura (1978) 100: A one-night shack-up at the Beverley Hills Hotel during a vacation.

3. a sexual relationship.

1960		‘Sheldon Lord’ College for Sinners 179: A college boy was supposed to go with a college girl. Any other arrangement was asking for trouble. A shack-up was fine, but he and Marge couldn’t be a shack-up as such.
1978		A. Maupin Tales of the City (1984) 59: For God’s sake! This isn’t a shack-up!
1982		A. Maupin Further Tales of the City (1984) 28: I paid for a weekend shack-up in Sierra City.