If you say that someone has shacked up with someone else or that two people have shacked up together, you disapprove of the fact that they have started living together as lovers. ( INFORMAL )
...the deserters who had shacked up with local women...
The Government was keen for people to get married rather than shack up...
It turned out she was shacked up with a lawyer in New York.
PHRASAL VERB : V P with n , pl-n V P , be V-ed P [ disapproval ]