shack up

Robert B.Parkerの "Painted Ladies"を読んでいます。
"It's a bitch, is what it's like," she said. "I mean, for sixteen years my mother lied through her teeth that he was dead. You know, she never even told me he sent money. You know that they were never married?"
"She told you they were?"
"Yeah, and that he died after I was conceived," she said. "Fact is, for crissake, she was shacking up with some guy who had no intention of marrying her, and when she got knocked up, he left."
"Tough on her, I guess," I said.

スラングが頻出する小説なので、"shacking up" も "knocked up" と同様にスラングと思います。"shack" は掘っ建て小屋の意味があるので、"shacking up" は家と関係のある意味ではないかと推測します。辞書で意味を確認します。
・Collins Dictionary: 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. The Government was keen for people to get married rather than shack up. :
・Cambridge English Dictionary: to start living in the same house as sexual partners, without being married: I hear Tony and Helen have shacked up together.
同じ家に棲むことから来ていましたね。