Green’s Dictionary of Slang

cased up adj.²

living with.

1936		J. Curtis Gilt Kid 51: [They] made arrangements to live together, get cased up properly in fact. [Ibid.] 33: He’d get cased up with some old bag and she’d take him for his lot.
1966		Trimble 5000 Adult Sex Words and Phrases.
1975		‘P.B. Yuill’ Hazell and the Three-card Trick (1977) 177: What was she doing cased up with a small bald cockney in his fifties?